The key to this four cheese truffle gnocchi gratin recipe, inspired by Osteria Santo Spirito in Florence, is to coat the gnocchi well in the white sauce, and then cook it in a shallow dish under the broiler so it gets nice and bubbly. The black truffle oil adds a decadent touch to this hearty dish.
Warm the milk in a saucepan over medium heat. In another saucepan, over medium low heat, stir butter until melted. Add the flour to the butter and stir constantly until it forms a roux, about 3 minutes.
Remove roux from heat and add the hot milk, whisking constantly. Season with a dash of kosher salt to taste. Return to the heat over medium and stir until the sauce is smooth and thickens, about 1 minute. Remove from heat and set aside. Stir in truffle oil.
Preheat oven to 400 F.
Cook the gnocchi in boiling water until done; strain.
Butter a large ovenproof baking dish. Add a layer of gnocchi, topped with half the white sauce and half the cheese. Top with the rest of the gnocchi, and add the remaining sauce and cheese.
Sprinkle with bread crumbs and bake until golden brown and bubbling, about 30-45 minutes. Start checking for doneness at about 30 minutes.
Serve, drizzled with more truffle oil if desired.
Notes
Tip: You can make your own Truffle Oil by heating 1 cup of good-quality olive oil in a saucepan until warm, and then stirring in 1 shaved truffle. Let steep until cool. Use immediately, or strain the oil and keep in an airtight container.