Place the asparagus in a small saucepan and cover with water. Bring to a boil; cook for 1-2 minutes. Drain and place in a 1-qt. baking dish coated with cooking spray. Top with fish; set aside.
In a small saucepan, saute onion in 1 tablespoon butter until tender. Stir in the flour, salt and pepper until blended. Gradually whisk in milk. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Remove from the heat; stir in cheese until melted. Pour over fish.
Melt remaining butter; stir in crackers and parsley. Sprinkle over cheese sauce. Bake, uncovered, at 350° for 20-25 minutes or until crumbs are golden brown and fish flakes easily with a fork.