In a small saucepan set over medium heat, melt the butter. Whisk in the flour until a paste forms and cook for a further minute or so. Stir in the mustard and milk. Continue whisking until a smooth sauce appears. this will only take a minutes. Add parsley, salt and pepper. Stir and cover until your fish is cooked.
Prepare the fish first by preheating the oven to 350 degrees F. In a medium sized baking dish, lay the fish out and set aside. Melt the the butter in a small skillet and add the garlic. Cook until sizzling and remove from heat and set aside. Combine the bread crumbs, lemon zest, parmesan, parsley, salt and pepper and mix well. Spoon the garlic butter over the fish and squeeze over the lemon juice. Top with the bread crumb mixture. Bake for about 25-30 minutes or until fish flakes apart.
Serve the fish with the sauce on the side!
*A note on times provided: appliances vary, any prep and/or cook times provided are estimates only.
Notes
Put the fish in the oven before you make the sauce. This will ensure it is both cooked at the same time.