No recipe, really. I've never used one. I just make it.
But... here's sort of how you do it...
Pick a casserole. The size of the casserole will determine how much cod and how many potatoes you need.
Slice or cube the potatoes. Boil until they are just starting to get soft. DO NOT overboil. Drain and set aside.
Either cut the cod into strips, or cube. Strips are better with sliced potatoes, cubes are better with cubed potatoes.
Finely chop a medium onion. Sweat in a pan with some butter and salt until it's starting to go soft and translucent. Add some garlic. Don't burn the garlic.
Make a medium white sauce. Add to the white sauce some powdered mustard and any other spices you want. Melt in as much shredded cheese as you want. Go slowly, you don't want it to break.
Stir in the onions and garlic.
Butter the casserole, and layer the potatoes and cod. After each layer add some of the white sauce, making sure that you keep enough white sauce for the top.
In a pan melt some more butter and foam it out. Add seasoned bread crumbs and stir to mix. Top the casserole with the bread crumbs.
Bake in a 325 degree oven until the potatoes are done and the center is heated through.
It's best if it sits for 10 to 15 minutes before you eat it.
Plate up, and sprinkle liberally with hot sauce.