Brush and scrub Jerusalem artichokes under cold running water. Cut them into 1/4inch slices and toss with lemon juice. Melt butter in a 4quart nonreactive saucepan. Add leek, carrot and Jerusalem artichokes with the lemon juice. Cover and cook over gentle heat for 20 minutes. Add 2 1/2 cups stock or water, 1 teaspoon salt and 1/4 teaspoon pepper. Cover and simmer for another 25 minutes. When cooked, remove from the heat and puree the soup with an additional 1/2 cup stock, or water and cream. Pass puree through a drum sieve. Return soup to a clean saucepan and reheat.
Ladle soup into warm soup bowls and dust each portion with fresh grated nutmeg.