In a nonstick skillet, cook mushrooms and onion in 3 tablespoons butter until tender.
Remove vegetables with a slotted spoon and set aside. Stir the flour, bouillon, salt, nutmeg and pepper into drippings until smooth. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in asparagus, pimientos, lemon juice and the mushroom mixture.
Pour into a 1-1/2-qt. baking dish coated with cooking spray. Melt remaining butter; toss with bread crumbs.
Sprinkle over top.
Bake, uncovered, at 350° for 35-40 minutes or until heated through.