Equipment
* optional
Ingredients
Vegetables
- 500 g russet potatoes, peeled
- 500 g turnips, peeled
Cream Sauce
- 400 ml heavy cream, cold
- 100 ml whole milk, cold
- 3 garlic, minced
- 4 fresh thyme, whole sprigs
- 8 g kosher salt
- 2 g black pepper, freshly ground
- 1 g ground nutmeg
Add-ins and Crust
- 30 g capers, drained and roughly chopped
- 50 g hazelnuts, toasted and roughly chopped
- 60 g parmesan cheese, freshly grated
- 15 g unsalted butter, softened
Nutrition (per serving)
Method
Preheat your oven to 190C/375F. Generously grease the interior of the gratin dish with the softened butter.
Using a mandoline slicer, slice the peeled potatoes and turnips into consistent 3 millimeter thick rounds. Keep them separate.
In a medium saucepan, combine the heavy cream, whole milk, minced garlic, thyme sprigs, kosher salt, black pepper, and nutmeg. Bring to a gentle simmer over medium heat, then immediately remove from the heat and discard the thyme sprigs.
Shingle the sliced potatoes and turnips in the prepared dish, creating overlapping layers. Alternate between potato and turnip layers, scattering the chopped capers evenly between the middle layers.
Pour the warm, infused cream mixture evenly over the layered root vegetables. Press down gently on the top layer with a spatula to ensure the liquid rises just to the top.
Cover the dish tightly with aluminum foil. Bake in the preheated oven for 45 minutes, or until the vegetables offer no resistance when pierced with a paring knife.
While the gratin bakes, combine the chopped hazelnuts and freshly grated Parmesan cheese in a small mixing bowl.
Remove the foil from the gratin. Scatter the hazelnut and Parmesan mixture evenly across the surface. Return the dish to the oven, uncovered, and bake for an additional 15 to 20 minutes until the top is deeply golden brown and the cream is bubbling visibly at the edges.
Remove the gratin from the oven. Let it rest at room temperature for at least 15 minutes before serving. This resting period allows the starches to set the cream sauce.
Chef's Notes
- Do not rinse the sliced potatoes. Their natural surface starches are the primary thickening agent for the cream sauce.
- A mandoline slicer is crucial for uniform thickness, ensuring the turnips and potatoes cook at exactly the same rate.
- Turnips carry a slight bitter edge when raw, but baking them slowly in heavy cream transforms them into something incredibly sweet and mellow. The bright acidity of the capers provides the perfect counterbalance to this richness.
- The 15 minute resting period is non-negotiable. Cutting into a gratin straight from the oven will result in a soupy texture, as the starches require a drop in temperature to gel and set the sauce properly.
Storage
Refrigerator: 4 days — Cover tightly with foil or plastic wrap to prevent drying.
Freezer: 1 month — Freezing may slightly alter the texture of the potatoes, but flavor remains excellent.
Reheating: Reheat portions in a 180C/350F oven until bubbly and warmed through, about 15 to 20 minutes.
