Preheat the oven to 200 degrees F (or the lowest setting your oven has). Heat oil in a deep fryer to 380 degrees F. Add the fish to the deep fryer and cook until golden brown. Remove the fish from the oil and drain on paper toweling. Sprinkle with salt. Wrap the hamburger bun in paper toweling and microwave for 10 seconds to steam it. Spread the tartar sauce on the cut sides of the bun. Place the fish patty on the bottom bun and top with the cheese. Place the top bun over the cheese. Wrap the filet-of-fish in a piece of foil and place in the oven. Let warm for 10 minutes at 200 degrees F then serve hot.