Cook potatoes until they are just soft, but not falling apart.
Cut potatoes into one inch cubes and cut fish into the same size pieces.
Season both with some garlic salt.
Pour melted butter over potatoes and remain over fish.
Lay everything out on a baking sheet. I prefer it all in one layer, this way it creates a better crust.
Bake it at 425 for 15 minutes, then turn on broil for a few minutes, just until it gets a nice crust. Serve it right away. If you plan to reheat it later, place a thin slice of butter on top to keep it moist and flavorful.