- Preheat oven to 180°C (350°F).
- Melt butter in a saucepan over medium heat and add flour. Cook, stirring with a whisk, for 1 minute. Add milk and thyme. Whisk until mixture thickens. Add 375 mL (1 1/2 cups) cheese and stir until smooth. Season generously with salt and pepper.
- Spread vegetables in a buttered rectangular ovenproof dish measuring 28 x 20 cm (11 x 8 in.). Season with Cayenne pepper, salt, and pepper. Cover vegetables with sauce and remaining grated cheese.
- Cover dish with aluminum foil and bake for 1 hour 30 minutes.
- Remove foil from dish and finish cooking by broiling for a few minutes, until cheese is golden.
You can replace Lustenberger by Vacherin or raclette cheese. This recipe can be easily adapted to whatever root vegetables you have on hand, including parsnips and celeriac.