Peel and dice the potatoes. Cut the onions and leeks into rings. Clean the Brussels sprouts and cut the stalk crosswise. Finely chop the garlic.
Heat the oil and sauté the onions and garlic in it. Add the leek and potatoes, finally the Brussels sprouts, deglaze with the broth and simmer the soup for about 20 minutes. Season to taste with salt, pepper and nutmeg.
Scoop the soup into plates to serve and garnish each plate with a spoonful of sour cream and chives.