Add all of the butter to a deep skillet. Add the spices, turn the heat to low and melt the butter. 

Let the spices steep in the butter on low for 15-30 minutes. 

Slide the fish fillets into the butter. You may need to do this in two batches. 

Turn the heat up slightly. If the butter starts spitting and bubbling, then the heat is too high. You want this to cook low and slow. 

Meanwhile, mix the mayo, ketchup, horseradish, vinegar, and Worcestershire in a bowl. Set aside. 

After about 10-15 minutes (depending on the size and thickness of the fillets) the fish should be done. Check by pressing a fork onto the thickest part of the fish. If it flakes easily, then it is done. 

Remove the fillets to a plate and season them lightly with salt and pepper. If some of the spices came up with the fillets, you can either brush them off, or leave them. I left them on and thought it was nice to get a bite of them every once in a while. 

You’ll be breaking them to fit onto the sandwiches, so don’t worry if they break while transferring them to the plate.

If you saved the skin, sprinkle them with salt and pepper. Spoon a tablespoon (15ml) or so of the poaching butter into another skillet. 

Lay the skins down into the skillet and cook over medium high heat, flipping once, until crispy like a potato chip. 

You can use this as a chef’s treat, a garnish for the sandwiches, or mix it into potato chips as a tasty side for the sandwich like I did.

Add another spoonful of the poaching butter to the frying pan. Add the bread slices (work in batches if they don’t all fit) and fry them until browned on one side only. 

Take them out and turn the toasted side upwards. Add 2 slices of the cheese onto one of the slices. Add half of the fish, breaking it up to fit onto the sandwich, stacking it high. Add a drizzle of the sauce, then add half of the sauerkraut. 

Add more sauce generously to another slice of bread (on the toasted side), then top the sandwich with it. Repeat for the other sandwich. 

Add another spoonful of the poaching butter to the skillet and fry the sandwiches like you would grilled cheese, over medium heat. Make sure they’re nice and browned, about 3-5 minutes per side. 

The butter can be strained and stored in the fridge to be used to poach more fish for a month or so. 

To serve, cut the sandwiches in half and serve either half, or a whole sandwich immediately, along with a side of pickles and potato chips or salad. Enjoy!