Preheat oven to 400 degrees. Spray a 9x13 pan with cooking spray. Arrange asparagus in dish, spacing out as flat as possible. In a saucepan on medium heat, melt 3 Tbsp. butter. Add flour and whisk constantly for 3 minutes or until butter just starts to turn golden. Whisk in half and half, garlic salt, and black pepper. Whisk until sauce just starts to thicken then remove from heat. Sprinkle in 1/2 cup parmesan and whisk until cheese is melted. Pour sauce evenly over asparagus. Bake, uncovered, at 400 degrees for 15-18 minutes, depending on the thickness of asparagus. While the casserole is baking, melt 1 Tbsp. butter. Mix in bread crumbs, stirring until evenly moistened, then stir in 1/2 cup remaining parmesan cheese. Halfway through baking, sprinkle the bread crumb topping evenly over the casserole then return it to the oven to finish baking. When asparagus has reached desired doneness, broil for 2-3 minutes or until bread crumbs are golden brown.