Break the tough bottoms from asparagus stalks and discard. Rinse the asparagus tips (reserve some for garnish) and place in large pot with chopped onion, margarine, chicken broth and skim milk.
Cook at medium heat until asparagus is very tender, 15 to 20 minutes. Add freshly ground black pepper to taste. Purée soup in batches in a blender or food processor until smooth.
Transfer to a bowl (use caution when blending hot liquids), and return to and whisk in yogurt and lemon squeeze of lemon juice. Garnish with asparagus tips.