Makes 2-4 servings.
In a medium saucepan, melt butter and mix in flour; whisk until a paste forms. Cook for 5 minutes, stirring constantly. Whisk in 1 cup (250 mL) milk, stirring constantly until smooth, approximately 2 to 3 minutes. Add remainder of milk, plus salt and pepper; whisk until it starts to thicken.
In a small bowl mix shredded cheese with bread crumbs. Set aside.
Place cod pieces in a baking dish. Cover with sauce and top with cheese and breadcrumb mixture. Bake at 375°F (190°C) for 20 to 25 minutes, or until golden on top.