Preheat Oven to 400˚F. Brush a 9x13 casserole dish with melted butter.
Place the fresh asparagus in the bottom of the buttered casserole dish.
In a saucepan over medium heart, melt 3 tablespoons of butter then add the 2 1/2 tablespoons of flour and whisk constantly for 1-2 minutes or until the butter just begins to turn a light golden brown color.
Whisk in the half and half, salt, garlic powder, and black pepper. Whisk until sauce begins to thicken then remove from heat and whisk in 1/2 cup parmesan cheese until melted and fully incorporated into the sauce.
Pour the sauce over the asparagus, leaving about 1 inch of the top and bottom of the asparagus uncovered.
Bake uncovered for 10 minutes.
In a small mixing bowl combine 1 tablespoon melted butter, the panko bread crumbs, and the 1/2 cup remaining parmesan cheese.
Sprinkle the bread crumb topping evenly over the casserole, then return it to the oven to bake for an additional 5-8 minutes, until the asparagus reaches your desired level of tenderness.
Set the oven to broil for 2-3 minutes or until bread crumbs are golden brown. Remove from oven and serve.