In a bowl mix together mayonnaise, relish, onion and salt. Stir until well combined. Set aside.
Cook the fish patties according to package directions, by baking in the oven or frying in a deep-fryer. For the best flavor, you will want to fry them.
If baking the patties, add the cheese slices over the fish squares before removing from the oven to melt the cheese. If frying the patties, you can add the cheese slice on top immediately after removing it from the oil, or by microwaving the sandwich on high for about 10 seconds.
Divide the tartar sauce and spread it evenly onto the bottom of the top half of each bun. Place the top half of the bun over the fish patty.