Heat the fat in a pot, dust with flour and sauté while stirring. Gradually stir in 150 ml water and milk, bring to the boil, stir in stock and simmer for 5 minutes over a mild heat.
Peel and wash the carrots, clean and wash the leek. Cut carrots into thin slices, leek into rings. Peel and wash potatoes and cut or slice them into thin slices. Arrange the vegetables in a small casserole dish.
Season the sauce with salt, pepper and nutmeg. Pour over the vegetables and sprinkle with cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 30-40 minutes. Garnish with chervil.