Peel 1 onion and cut into fine cubes. Wash marjoram, shake dry and chop 2 stems finely. Set the rest aside. Melt 1 tablespoon butter in a pot. Sauté onion until translucent while stirring.
Add flour and sweat briefly. Add wine and stock while stirring. Let simmer for about 5 minutes. Add cream and chopped marjoram, season to taste with salt, pepper and sugar. Keep warm.
Wash the chicken fillets and dab dry. Season with salt and pepper. Heat 1 tablespoon of oil in a large coated pan. Fry the fillets in it at medium heat while turning for about 10 minutes.
Meanwhile peel 1 onion, halve and cut into strips. Clean, wash and halve the Brussels sprouts. Clean and halve the mushrooms. Cook the sprouts in boiling salted water for about 5 minutes. Drain and rinse with cold water.
Heat 1 tablespoon of oil in a frying pan. Fry the mushrooms, onion and Brussels sprouts for 3-4 minutes while turning them, season with salt and pepper.
Prepare the spaetzle in boiling salted water according to the instructions on the packet. Drain. Put back into the pot with 1 tsp butter and toss. Finely chop the remaining marjoram, except for a few leaves for garnishing.
Remove the fillets from the pan and turn them in the chopped marjoram. Cut into slices and arrange on plates with vegetables, spaetzle and sauce. Garnish with marjoram.