Peel and dice the potatoes. Rinse the Brussels sprouts in a colander under water. Braise both with the caraway seeds in 20 g of heated butter. Top up with broth and cook, covered, over a mild heat for 20 minutes.
Let the milk get hot with salt, pepper and nutmeg. Add the potatoes and Brussels sprouts and mash well with a potato masher. Serve the puree garnished with the melted onions.