On a wire rack, place the cod filets. Pat them dry, season both sides with salt, and let them sit for 20 minutes.
In a bowl, combine mayo, onions, capers, gherkins, dill, and lemon juice. Incorporate a tablespoon of caviar into the tartar sauce. Adjust seasoning with additional lemon juice if necessary.
On a plate, mix the all-purpose flour with your chosen rub. Dredge the cod filets in this flour mixture and return them to the wire rack.
In a mixing bowl, combine seasoned flour, additional flour, salt, and rub. Gradually whisk in the beer until the batter reaches a slightly thick consistency.
In a large pot, preheat enough oil for deep drying. Dip each cod filet into the batter, ensuring they're fully coated, then fry in the hot oil for 3-4 minutes or until golden and crispy. Return the fried filets to the wire rack and top each with a slice of American cheese.
In a pan, melt some butter. Toast both sides of the brioche bun until golden.
To assemble: On the bottom half of the toasted brioche, spread a generous layer of the tartare sauce mixed with caviar. Place the battered cod filet on top, followed by more tartare sauce and a teaspoon of caviar. Cap off with the top half of the brioche bun. Serve and enjoy!