Peel and wash the carrots and cut them diagonally into slices. Clean and wash leek and cut into rings. Peel and wash the potatoes and also cut them into slices. Pour everything into a large greased casserole dish.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Peel and chop garlic, heat with cream. Season with salt, pepper, nutmeg and 1 tsp. thyme. Pour cream over the vegetables. Bake in a hot oven for about 45 minutes.
In the meantime roughly chop the hazelnuts. After about 40 minutes baking time, sprinkle on the gratin and bake until done.