In a saucepan over low heat, or in a double boiler, combine the cheese and ale and cook until the cheese melts, 5 to 7 minutes. Do not allow the mixture to boil. Add the flour, mustard, and breadcrumbs and cook until the mixture comes away from the sides of the pan, about 5 minutes. Season with salt and pepper and allow to cool.

When cool, transfer to a food processor or blender and process until smooth. With the motor running, slowly add the egg and egg yolk. Transfer to a small bowl, cover and refrigerate for 30 minutes.

Preheat oven to 350°F. Transfer cheese mixture to a pastry board and roll out to a smooth thickness of about 1/4-inch. Lay haddock pieces into a buttered ovenproof casserole dish, cut pieces of cheese mixture to fit on top of haddock (trim the edges), and bake, uncovered, for about 10 minutes, or until haddock is heated through. Place under the broiler to brown the crust.