In a large pot, combine onion, thyme potato and stock. Cover and simmer until potato is nearly tender, about 10 minutes.
Meanwhile, cut asparagus into 1-inch (2-cm) lengths. Add to pot, cover and simmer until asparagus is tender, about 5 minutes. Allow to cool slightly, and in a food processor (or blender) purée with lemon juice. Stir in milk and yogurt. Season with salt and freshly ground pepper to taste.
To serve hot, reheat carefully, and do not boil. (Also, delicious cold.)
Per serving: 158 calories, 10 g protein, 2 g fat (1 g saturated fat), 29 g carbohydrates, 4 g fibre, 5 mg cholesterol, 82 mg sodium