Batch-Cooked Sweet Potato & Potato Gratin with Garlic & Thyme

There’s something quietly magical about pulling a bubbling, golden-crusted gratin from the oven on a Tuesday night—especially when you know the hardest work was done last Sunday while the playlist was still on shuffle and the coffee was hot. I started batch-cooking this particular gratin three winters ago, the year we renovated the kitchen and the only working appliance was the oven. I’d slice, simmer, and stack pans of potatoes on Sunday afternoon, then slide them into the fridge so we could reheat portions all week. The sweet potatoes caramelise at the edges, the regular potatoes stay velvety, and the thyme-garlic cream winds its way into every crevice. It’s comfort food that doubles as meal-prep heroics, and it feels fancy enough for company even when you’re eating it in pyjama bottoms.

Why This Recipe Works

Two-potato power: Sweet potatoes bring natural sugars that caramelise beautifully, while Yukon Golds stay creamy and hold their shape.
Infused cream: Simmering garlic and thyme in the cream first extracts maximum flavour without chunky bits.
Batch-friendly: Assemble two 8-inch square pans—bake one now, freeze the other uncooked for up to 2 months.
Reheats like a dream: Cream re-emulsifies in the oven so leftovers taste freshly baked, not dried out.
No cheese required: The starch from the potatoes thickens the cream so you get a silky sauce without a flour roux or dairy overload.
Make-ahead holidays: Slice everything the day before; store submerged in acidulated water so potatoes don’t brown.

Ingredients You’ll Need

Quality ingredients make the difference between a watery gratin and one that slices cleanly yet spoons away like velvet. Here’s what to look for:

Sweet potatoes: Choose firm, medium-sized garnets or jewels with tight, unblemished skins. Avoid the mega-sized ones—they tend to be fibrous. You’ll need about 900 g total.
Yukon Gold potatoes: Their medium starch content means they hold shape but still absorb cream. Russets can work in a pinch, but they’ll break down more and give a fluffier texture.
Heavy cream: 35 % fat minimum. Lower-fat liquids split under long heat. If you must lighten, swap 25 % of the cream with whole milk, not more.
Garlic: Fresh cloves, smashed to release allicin. Jarred minced garlic tastes tinny here.
Fresh thyme: Woody stems infuse the cream; leaves stay behind so no one gets a twiggy surprise. In summer, substitute lemon thyme for a brighter note.
Nutmeg: Just a whisper—freshly grated. Pre-ground tastes dusty.
Butter: Unsalted, for greasing and dotting. Salted butter can curdle cream.
Sea salt & white pepper: White pepper keeps the colour pristine; black specks can look like burnt bits.

Optional but lovely: a bay leaf in the cream, or 30 g finely grated Gruyère if you want a cheese crust. For a vegan version, substitute full-fat coconut milk and plant butter; the flavour will be sweeter and mildly coconutty—delicious with Caribbean mains.

How to Make Batch-Cooked Sweet Potato & Potato Gratin with Garlic & Thyme

Infuse the cream

In a small saucepan combine 500 ml heavy cream, 3 smashed garlic cloves, 4 thyme sprigs, ½ tsp freshly grated nutmeg, 1 tsp sea salt, and ¼ tsp white pepper. Bring just to a bare simmer over medium-low—look for bubbles around the edge—then remove from heat, cover, and steep 15 minutes while you prep vegetables. This step extracts herbal oils and mellows the garlic.

Slice potatoes uniformly

Peel 450 g sweet potatoes and 450 g Yukon Golds. Using a mandoline set to 3 mm (⅛-inch), slice directly into a bowl of cold water with 1 tbsp white vinegar to prevent oxidation. Uniform thickness guarantees even cooking; 3 mm is thin enough to layer but thick enough to hold structure after baking.

Build your batch pans

Butter two 8-inch square aluminium pans (or one 9×13). Drain potatoes and pat dry. Alternate stacks: one slice sweet potato, one slice Yukon Gold, standing on their edges like fallen dominoes. This shingled look isn’t just pretty; it exposes maximum surface area to cream. Continue until pans are tightly packed.

Strain and season cream

Remove thyme stems and garlic from cream. Taste; add more salt if needed—potatoes are bland magnets. The cream should be well-seasoned because it seasons everything else.

Pour and press

Divide infused cream between pans. Press down so liquid rises just to the top layer. Dot surface with 15 g cold butter cubes. Cover pans tightly with foil labelled “Gratin – bake 30c 375f” so future-you knows what to do.

Bake now or freeze

To bake immediately: place foil-covered pan on a rimmed tray (catches drips) and bake at 190 °C / 375 °F for 30 minutes. Remove foil, bake 25–30 minutes more until top is blistered and cream is bubbling thickly. A knife should slide through with zero resistance.

Rest before serving

Let gratin stand 10 minutes; starch sets and cream settles from a slosh to a silken sauce. Serve straight from the dish or cut into tidy squares for plating.

Cool, portion, store

For meal-prep, cool completely, then cut each pan into 6 squares. Refrigerate in airtight boxes up to 4 days or freeze up to 2 months. Reheat covered at 175 °C / 350 °F for 15 minutes (from fridge) or 25 minutes (from frozen). Cream re-emulsifies and edges crisp again.

Expert Tips

Dry potatoes = creamy sauce

After soaking, spin slices in a salad spinner or pat with kitchen roll. Excess water dilutes cream and causes curdling.

Use an oven thermometer

Cream can break above 195 °F. If your oven runs hot, lower temp 10 °C and extend time 5–7 minutes.

Double-deck for parties

Bake in a spring-form lined with parchment; remove sides for a towering gratin cake. Slice into wedges for wow-factor.

Overnight method

Assemble, cover, refrigerate up to 24 h. Add 10 extra minutes covered bake time straight from fridge.

Crisp top hack

Dust surface with 15 g finely grated Parmesan after removing foil. Broil 2 min at end for lacquered crust.

Scale by weight

Potato density varies. Weighing ensures cream ratio stays correct; you need roughly 1 ml cream per 1 g potatoes.

Variations to Try

Smoky kale & gruyère: Tuck 2 cups shredded lacinato kale and 60 g smoked Gruyère between layers. Adds iron and a bonfire note.
Asian-spiced: Swap thyme for 1 tsp Thai red curry paste, add 1 tsp lime zest to cream. Finish with crispy shallots.
Turnip & parsnip remix: Replace half the potatoes with parsnip and baby turnip for a lower-carb, peppery spin.
Single-serve stacks: Use a jumbo muffin tin; reduce bake time to 20 minutes covered, 10 uncovered. Perfect for brunch buffets.
Truffle deluxe: Drizzle 1 tsp white truffle oil into the strained cream. A little goes far; too much overwhelms the sweet potatoes.

Storage Tips

Cool portions within 2 hours of baking to minimise time in the bacterial danger zone. Refrigerate in shallow containers—deep tubs hold heat and spoil faster. Frozen gratins stay prime for 2 months; beyond that, cream can separate and potatoes turn grainy.

When freezing, wrap the entire pan in two layers: first plastic wrap (touching the surface to prevent ice crystals), then foil. Label with recipe name, date, and baking instructions so helpful housemates don’t mistake it for plain potatoes. Thaw overnight in fridge or bake straight from frozen—just add 15 minutes to the covered phase.

Microwave reheating works for single portions (900 W, 90 seconds, covered), but the top won’t crisp. For best texture, reheat in oven or air-fryer at 180 °C for 5–6 minutes.

Frequently Asked Questions

Half-and-half risks curdling because it’s only 10–12 % fat. If you must, swap 25 % of the cream with whole milk and add 1 tsp cornstarch to stabilise. Texture will be less rich but acceptable.

Graininess happens when cream boils too rapidly or is frozen after baking. Keep oven under 190 °C, and if freezing, do so before baking for best texture.

Slow cookers don’t evaporate enough liquid, so you’ll get soupy potatoes. If you must, layer as directed, cook on LOW 4 hours, then transfer to oven for 20 minutes uncovered to reduce sauce.

Think roasted proteins that share the oven: herb-crusted salmon, citrus chicken thighs, or a mushroom and lentil loaf. The sweetness complements spicy harissa lamb or smoky BBQ tofu equally.

A knife should meet zero resistance at centre, and cream should be thick enough that when you tilt the pan, it lazily holds a line rather than rushing like milk.

Absolutely—use a deep 10×15 roasting pan and add 10 minutes covered time. The critical ratio is cream volume to potato weight; keep 1 ml cream per 1 g potatoes and you’re safe.