Drain asparagus, reserving liquid. Add enough milk to liquid to measure 4 cups; set aside. In food processor or electric blender, puree asparagus; set aside. In 3-quart saucepan over medium heat, cook onions in butter until soft but not brown. Stir in flour, salt, and bouillon. Add milk mixture and asparagus, stirring until smooth. Cook, stirring constantly, until mixture boils. Cook and stir 1 minute longer. Remove from heat; serve hot. Garnish with cooked asparagus spears if desired. Yields 5 servings, 1 cup each.