Melt butter in a saucepan over medium heat. Add flour and stir until well combined. Cook for 1 minute or until bubbly. Remove from heat and slowly whisk in milk. Return to heat and stir constantly until mixture comes to the boil and thickens.
Stir in 1 packet Gravox Cheese Finishing Sauce and 1/4 cup cheese. Reduce heat to low and simmer for 2 minutes. Stir through dill. Remove from heat. Place asparagus into a bowl and cover with boiling water. Stand for 2 minutes. Drain.
Lay fish into a large shallow ceramic dish. Top with asparagus. Pour over sauce. Combine remaining cheese and breadcrumbs in a small bowl. Sprinkle over fish and asparagus. Bake for 15 minutes or until topping is golden. Serve immediately, with extra Gravox Cheese Finishing Sauce and garnished with extra dill.