Holiday Truffle and Mushroom Potato Gratin
A decadent potato gratin layered with sautéed mushrooms, creamy gruyère, and finished with a drizzle of truffle oil. Tender potatoes bake until golden and bubbly in a rich cream sauce, while earthy mushrooms add depth and umami. This elegant side is French-inspired yet surprisingly approachable—perfect for holiday dinners or special occasions.
- Prep
- 30 min
- Cook
- 50 min
- Wait
- 15 min
- Total
- 1 hr 35 min
- Makes
- One 9×13-inch (23×33 cm) gratin, 8 servings
Steps
Before you start
- Preheat oven to 375 °F (190 °C) 15 minutes before baking
- Have all ingredients measured and the gratin dish ready
- Use a mandoline or sharp knife to slice potatoes uniformly (⅛ inch / 3 mm thick)
- Choose cremini, shiitake, or a mix of mushrooms for best flavor
Sauté mushrooms with truffle
- In a large heavy skillet, melt 2 tablespoons (28 g) of butter over medium-high heat.
- Add the sliced shallots and cook, stirring occasionally, for 3–4 minutes until softened and golden.
- Add the minced garlic and cook 1 minute more until fragrant.
- Increase heat to medium-high, add the mushrooms, and sauté for 8–10 minutes, stirring occasionally, until they release their moisture and the liquid evaporates.
- The mushrooms should be tender and beginning to brown.
- Drizzle with 1 tablespoon (15 ml) of truffle oil, stir to coat, and season generously with salt and pepper.
- Transfer to a plate and set aside to cool slightly.
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Ready when: Mushrooms are tender and liquid has evaporated; they should begin to brown at the edges
Tip: Don't crowd the mushrooms—cook in batches if needed—so they brown rather than steam. This concentrates their umami flavor.
Prepare cream sauce base
- In a medium saucepan, combine the heavy cream (600 ml) and whole milk (240 ml).
- Heat over medium heat, stirring occasionally, until small wisps of steam rise from the surface—do not boil.
- Whisk in the salt, black pepper, and nutmeg to taste.
- Remove from heat and set aside.
- The sauce should be warm and well-seasoned.
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Ready when: Steam rises from the surface; cream is heated through but not boiling
Tip: Heating the cream and milk ahead of time helps the potatoes cook more evenly and prevents the gratin from developing a watery layer.
Assemble the gratin
- Butter a 9×13-inch (23×33 cm) gratin dish with the remaining 2 tablespoons (28 g) of butter.
- Begin layering: spread about one-quarter of the sliced potatoes in an even layer on the bottom.
- Season lightly with salt and pepper.
- Scatter about one-third of the sautéed mushroom mixture over the potatoes, then sprinkle with about ⅓ cup (33 g) of the grated gruyère and 1 teaspoon of fresh thyme leaves.
- Repeat this layering two more times (potatoes, mushrooms, cheese, thyme), then top with the remaining potatoes (roughly one-quarter of the total), season lightly, and top with the remaining ½ cup (50 g) gruyère and ½ cup (50 g) parmigiano-reggiano (if using).
- Pour the warm cream sauce slowly and evenly over the top, tilting the dish and using a spatula to gently encourage the cream to seep down between the layers.
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Ready when: Layers are even and cream sauce is distributed throughout; no visible dry spots on top
Tip: Slice potatoes uniformly so they cook evenly. If any are thicker than ⅛ inch, they may remain firm in the center while outer layers overcook.
Bake until golden and tenderOven 375°F / 190°C
- Place the gratin in a preheated 375 °F (190 °C) oven on the middle rack.
- Bake uncovered for 45–50 minutes until the top is deep golden brown and a sharp knife inserted into the center of the gratin meets just slight resistance—the potatoes should be tender and the cream should bubble gently at the edges.
- If the top begins to brown too quickly (after 25–30 minutes), loosely tent with aluminum foil and continue baking.
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Ready when: Top is deep golden brown; sharp knife inserted meets slight resistance; cream bubbles gently at edges
Tip: Tent with foil early if browning too fast—the cream needs time to thicken and the potatoes need to soften. Check doneness with a sharp knife, not a fork, which may tear the potatoes.
Rest and finish with truffle oil
- Remove the gratin from the oven and let it rest at room temperature for 10–15 minutes.
- This allows the cream to set slightly, making the gratin easier to serve and more stable on the plate.
- Just before serving, drizzle the remaining 2 tablespoons (30 ml) of truffle oil in a light, even pattern across the top of the gratin.
- This final truffle oil drizzle adds an aromatic, luxurious finish.
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Ready when: Gratin is warm throughout and has firmed slightly; top is still golden
Tip: Add the final truffle oil drizzle just before serving to preserve its delicate aroma. If added too early, it may become muted as the dish cools.
Good to know
Tips
- Slice potatoes as uniformly as possible—even a ⅛-inch (3 mm) thickness matters. A mandoline slicer is worth the investment for gratins, or use a sharp chef's knife and work carefully.
- Warm the cream and milk before pouring. This reduces baking time and ensures even cooking of the potatoes from the start, preventing a watery layer at the bottom.
- Don't skip the mushroom sauté step. Cooking them first concentrates their flavor and allows them to release and reabsorb moisture, preventing a soggy gratin.
- Season each layer lightly as you build the gratin—don't save all seasoning for the sauce. This ensures flavors are distributed throughout, not just at the top.
Make ahead
You can assemble the gratin up to 4 hours ahead of baking. Cover it with plastic wrap and refrigerate. Add 5–10 minutes to the baking time if baking directly from cold. Alternatively, bake the gratin 1 hour ahead, let it rest at room temperature, and reheat gently in a 325 °F (163 °C) oven for 15–20 minutes until warmed through before adding the final truffle oil drizzle.
Storage & reheating
Store leftover gratin covered in the refrigerator for up to 3 days. Reheat gently in a 325 °F (163 °C) oven for 15–20 minutes until warmed through (do not reheat in the microwave, which can make the potatoes rubbery). Do not freeze—the cream-based sauce separates when thawed. Omit the final truffle oil drizzle until just before serving for best flavor.
Substitutions
- gruyère cheese, finely grated → Emmental, Comté, or sharp aged white cheddar. These cheeses melt smoothly and have a similarly nutty, complex flavor. Use the same amount. Avoid pre-shredded cheese, which contains anti-caking agents and won't melt as creamily.
- black truffle oil → 1 tablespoon (15 ml) white truffle oil for sautéing mushrooms; finish with 1 tablespoon (15 ml) high-quality extra-virgin olive oil mixed with a pinch of fleur de sel. White truffle oil has a lighter, more delicate aroma than black. This substitution gives an earthy, elegant finish with less intensity.
- mixed fresh mushrooms (cremini, shiitake, oyster) → All cremini, all shiitake, or all porcini (if using dried, rehydrate in warm water and chop). Any choice will work well; use what's available. Dried porcini add deeper, more concentrated umami. If using dried, add the strained soaking liquid to the cream sauce for extra depth.
- russet potatoes, peeled → Yukon gold potatoes. Yukon golds are creamier and won't break apart as easily, though russets are traditional for gratins. Yellow potatoes may cook slightly faster, so check at 40 minutes.
