Wash the potatoes and cook covered for about 20 minutes. drain, quench and peel
Clean or peel and wash the vegetables. Cut broccoli and cauliflower into florets, cut carrots into slices. Peel and finely dice onion
Bring 1/2 l water and broth to the boil. Cover the cauliflower and carrots and cook for 10-12 minutes. Cook broccoli for about 5 minutes. Drain vegetables. Collect broth, measure 400 ml
Grate cheese. Sauté onion in hot fat. Sweat flour in it. Deglaze with stock and cream, bring to the boil. Melt half the cheese while stirring. Season sauce to taste
Put the potatoes and vegetables in a large casserole dish. Pour sauce over it and sprinkle with the rest of the cheese. Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for about 20 minutes. Garnish with chervil