In a skillet, sauté the asparagus and onion in 1 tablespoon of butter until crisp tender, about 8 minutes. Transfer to an ungreased 1 1/2-quart baking dish. In a saucepan, melt 2 tablespoons butter. Stir in flour until smooth, gradually add milk. Bring to a boil, cook and stir 2 minutes or until thickened. Reduce heat. Add cream cheese, salt and pepper, stir until cheese is melted. Pour over the vegetables. Sprinkle with Cheddar cheese. Melt remaining butter, toss with breadcrumbs and sprinkle over casserole. Bake uncovered at 350° F for 30 - 40 minutes or until heated through. Yields 4 - 6 servings.