Mince the leeks and onion. Stir and sauté them 3 minutes in 2 tablespoons butter. Pare, and slice very fine the potatoes and add 4 cups stock. Simmer the vegetables, covered, 15 minutes or until tender. Put them through a very fine sieve food mill or blender. Add the 1 to 2 cups cream, mace, salt and pepper to taste and garnish with chopped watercress or chives. Makes about 8 cups.