Creamy Celeriac and Potato Gratin with a Toasted Hazelnut Crust Step-by-step instructions

Before you start

Before beginning the assembly, ensure the oven is fully preheated to 375°F (190°C) so the gratin can start cooking immediately upon entering. Prepare a workspace by cleaning the counter thoroughly, as the mandoline requires a stable surface.

Have all ingredients measured and the cheese grated in advance, as the layering process flows best when everything is within easy reach. Peel and slice the vegetables carefully to avoid waste and ensure they are ready to be layered into the ceramic dish.

Instructions

Aromatic Cream Infusion

1. 1Simmer the aromatics
   Place the heavy cream, smashed garlic cloves, fresh thyme, and bay leaf into a large saucepan. Bring the mixture to a gentle simmer over medium heat, ensuring it does not boil over. Let the cream infuse with the aromatics for about five minutes to develop a deep, savory flavor profile before adding the vegetables.
   Look for The cream is fragrant and lightly steaming.
   Tip Keep the heat low to prevent the cream from scorching at the bottom of the pan.

The cream should be gently simmering with the garlic, thyme, and bay leaf. The kitchen should smell aromatic and savory. Ensure the cream is not boiling vigorously to maintain its smooth, velvety texture. This phase is complete when the garlic has softened and the herbs have imparted their essential oils into the liquid, creating a fragrant base for the gratin.

Layering and Assembly

2. 2Layer the gratin
   Peel and slice the celeriac root and Yukon Gold potatoes into uniform 2mm discs using a mandoline for precision. Arrange the slices in the baking dish, alternating the celeriac and potatoes to create distinct, beautiful layers. Pour the warm, infused cream mixture over the vegetables, ensuring every layer is thoroughly coated for even cooking.
   Look for The vegetables are tightly packed and submerged in the cream.
   Tip Aim for very thin slices to ensure the gratin cooks through evenly and becomes perfectly tender.

The baking dish should be filled with uniform layers of celeriac and potato discs, fully submerged in the warm cream. Ensure the vegetables are arranged tightly to create a stable structure. The dish is ready for the oven when the cream level is just below the top layer of vegetables, ensuring consistent moisture distribution during the baking process.

Baking to Golden Perfection

3. 3Top and bake
   Distribute the 180g of grated Gruyère cheese evenly over the layered vegetables. Place the dish into a preheated oven at 375°F (190°C) and bake for 35 minutes. Carefully remove the dish, sprinkle the crushed roasted hazelnuts and the additional 50g of Gruyère over the top, then return to the oven for a final 10 minutes until the crust is golden and blistering.
   Look for The top is deep golden brown and the cream has thickened into a velvet consistency.
   Tip If the top browns too quickly, cover it loosely with aluminum foil for the final few minutes of baking.

The gratin is ready when the surface is blistered, golden brown, and the hazelnuts are toasted. The cream should be thick and bubbling around the edges.

Check that the vegetables are tender by inserting a knife into the center; it should slide through without any resistance. The final result should be a beautiful, cohesive dish with a distinct, crunchy top layer.

Common mistakes

A common mistake is slicing the vegetables too thickly, which prevents them from softening uniformly and results in a gritty texture. Another frequent error is failing to infuse the cream sufficiently; skipping the simmering step with the aromatics leaves the dish tasting flat.

Finally, do not rush the baking process by increasing the oven temperature, as this will burn the cheese topping before the root vegetables have had sufficient time to cook through to the center.

How to tell it’s done

The gratin is done when the vegetables are completely tender and offer no resistance when pierced with a small knife. The edges of the cream mixture should be bubbling, and the top layer of Gruyère and hazelnuts should be deeply golden brown and crispy.

Allow the gratin to rest for at least ten minutes after removing it from the oven, as this helps the cream settle and makes the final portions easier to scoop and serve beautifully.