Wash the potatoes thoroughly, cover and cook in plenty of boiling water for about 20 minutes. Drain potatoes, rinse and peel. Let potatoes cool down. In the meantime peel and finely dice the onions.
Set aside 1 tablespoon of onions. Heat the fat in a saucepan and fry the remaining onions while stirring. Dust with flour and continue to fry for 2-3 minutes. Add milk and white wine while stirring and bring to the boil.
Then continue to simmer for about 5 minutes at low heat while stirring. Season sauce with salt, pepper and nutmeg to taste. Grease a high gratin or casserole dish. Cut the potatoes into slices and layer them in layers, alternating with the sauce.
Pour cream over it and sprinkle with cheese. Bake in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 5) for 25-30 minutes until golden brown. Clean lamb's lettuce, wash thoroughly several times and drain well.
Mix the remaining onions, vinegar, a little salt and pepper. Fold in the oil. Mix the vinaigrette with the lamb's lettuce just before serving. Serve onion potatoes with Bündnerfleisch and salad.