Melt the butter in a saucepan and sauté the onions and Brussels sprouts in it. Pour in the broth and cook covered over medium heat for about 10-15 minutes.
Remove half of the Brussels sprouts and puree the rest in the broth. Possibly add a little more broth. Season to taste with salt, pepper and nutmeg, refine with crème fraîche. Add the rest of the Brussels sprouts, heat them up.
Suitable as an insert: croutons, wiener sausages, cooked ham, smoked pork or fried breakfast bacon.