Potato and Celery Root Gratin
A classic French-inspired gratin layering thin slices of potatoes and celery root in a rich cream sauce, topped with melted Gruyère cheese. The vegetables become tender and creamy while the cheese browns and crisps on top. Elegant enough for a dinner party but simple enough for a weeknight side.
- Prep
- 25 min
- Cook
- 50 min
- Wait
- 10 min
- Total
- 1 hr 25 min
- Makes
- One 9×13-inch (23×33 cm) gratin, 6 servings
Steps
Before you start
- Preheat your oven to 375°F (190°C) so it reaches temperature while you prep the vegetables
- Use a sharp knife or mandoline to slice vegetables uniformly (about 1/8 inch thick) so they cook evenly
- Have your gratin dish ready and your cream mixture prepared before layering to prevent vegetables from browning
Prepare the baking dish
- Lightly butter a 9×13-inch (23×33 cm) baking dish.
- Set aside.
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Tip: A well-buttered dish prevents sticking and helps create crispy edges.
Make the cream sauce
- In a medium saucepan, melt 2 tablespoons (28 g) butter over medium heat.
- Whisk in the flour and cook, stirring constantly, for 1 minute until lightly golden—this cooks out the raw flour taste.
- Gradually whisk in the heavy cream and whole milk until smooth, breaking up any lumps.
- Continue whisking and cooking for 3–4 minutes until the sauce thickens enough to coat the back of a spoon.
- Stir in the minced garlic (if using), salt, pepper, and nutmeg.
- Remove from heat and fold in 1 cup (100 g) of the grated Gruyère.
- Taste and adjust seasoning.
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Ready when: Sauce coats a spoon and tastes creamy and well-seasoned
Tip: Whisk constantly to prevent lumps and scorch marks. The roux (butter-flour mixture) shouldn't brown.
Layer vegetables and sauce
- Spread a thin layer of cream sauce (about 1/4 cup / 60 ml) on the bottom of the prepared baking dish.
- Arrange slightly overlapping potato slices in a single layer.
- Season lightly with salt and pepper.
- Add a layer of slightly overlapping celery root slices.
- Season again.
- Drizzle with about 1/4 cup (60 ml) of cream sauce, spreading it gently so the vegetables don't shift.
- Repeat, alternating potatoes and celery root, layering sauce every two vegetable layers, until all vegetables and most of the sauce are used.
- The top layer should be vegetables, finished with a final pour of remaining sauce (about 1/2 cup / 120 ml) so the top layer is moistened.
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Ready when: All vegetables are layered and sauce covers the top
Tip: Overlapping vegetable slices like roof shingles prevents gaps and ensures even cooking. Don't compress the layers.
Top with cheese and bakeOven 375°F / 190°C
- Sprinkle the remaining 1/2 cup (50 g) of grated Gruyère evenly over the top.
- Place the baking dish on the center rack of a 375°F (190°C) oven and bake, uncovered, for 45–50 minutes.
- The gratin is ready when the cheese is golden brown and bubbling at the edges, and a fork inserted into the potatoes encounters no resistance—the vegetables should be completely tender and the top creamy.
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Ready when: Cheese is golden brown and bubbling at edges; potatoes are fork-tender
Tip: If the cheese is browning too quickly, loosely tent with foil for the last 10–15 minutes. It should brown, not char.
Rest and serve
- Remove the gratin from the oven and let rest at room temperature for 10 minutes.
- This allows the cream to set slightly so the gratin holds its shape when plated.
- Serve warm, spooning creamy vegetables onto each plate.
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Ready when: Gratin has cooled slightly but is still warm and creamy
Tip: Resting prevents the gratin from breaking apart when you serve it. The cream will thicken more as it cools.
Good to know
Tips
- Slice both potatoes and celery root uniformly using a mandoline or sharp knife set at 1/8 inch (3 mm)—uneven slices cook at different rates, leaving some hard and some mushy.
- Celery root oxidizes quickly once cut, so slice it last and layer it immediately, or hold the slices in cold water and drain well before using.
- The cream sauce should be thick enough to coat vegetables but loose enough to pour—if it's too thick, warm it gently and whisk in a splash more milk.
- For a lighter gratin, replace half the heavy cream with whole milk or even vegetable stock, though the result will be less rich.
Make ahead
Prepare the cream sauce and slice the vegetables up to 4 hours ahead; store the sauce covered in the fridge and the vegetables in separate bowls of cold water (change the water once). Drain the celery root slices well and pat dry before layering to avoid excess water. Assemble and bake the gratin no more than 1 hour before serving for the best texture.
Storage & reheating
Refrigerate leftovers in an airtight container for up to 3 days. To reheat, cover with foil and warm in a 325°F (165°C) oven for 20–25 minutes until heated through (or cover loosely and microwave for 2–3 minutes). The gratin does not freeze well due to the cream sauce's texture.
Substitutions
- Gruyère cheese → Emmental or Comté cheese. Both have a similar nutty flavor and melt beautifully; use the same amount.
- celery root (celeriac) → parsnips or a mix of parsnips and potatoes. Parsnips offer a sweeter flavor; slice the same thickness and add an extra 5 minutes to baking time if parsnips are thick.
- heavy cream → crème fraîche or sour cream mixed with a little milk. Use 1 cup (240 ml) crème fraîche plus 1/2 cup (118 ml) milk. The sauce will be slightly less rich but still creamy.
- Yukon Gold potatoes → waxy potatoes like red potatoes or fingerlings. They hold their shape well when sliced thin; peel or leave skin on for a rustic texture.
