In a saucepan, melt butter over medium heat. Stir in flour until smooth. Gradually add milk, stirring constantly until thickened. Season with salt and pepper.
Place fish fillets in a greased baking dish. Pour the sauce over the fish.
Sprinkle shredded cheese and breadcrumbs on top.
Bake for 25-30 minutes, or until the fish is cooked through and the top is golden brown.
Let it cool slightly before serving.
Notes:
You can use any white fish you like. Serve with a side of vegetables or a fresh salad.