In saucepan, combine potato, onion, and water or chicken stock; cover and simmer until potato is nearly tender, 5 to 10 minutes.
Meanwhile, cut asparagus into about 1 1/2-inch lengths (4 cm).
Add to potato mixture; cover and simmer for 5 minutes or until asparagus is tender.
Using slotted spoon, remove asparagus tips and let cool in cold water to prevent further cooking.
Drain and reserve for garnish.
In food processor or blender, puree asparagus-potato mixture; add lemon juice.
Pour into bowl; cover and refrigerate until chilled.
Stir in milk and yogurt; season with salt, pepper and nutmeg to taste.
Serve cold or reheat.
Garnish each serving with reserved asparagus tips.
Makes 6 servings, 3/4 cup/1 75 ml each.