Make the tartar sauce at least 1 hour ahead. Combine the mayonnaise, diced dill pickle, dill, Worcestershire sauce, Dijon mustard, lemon juice, and black pepper. Refrigerate until ready to serve.
Place the cod loins on a sheet pan and pat completely dry. Season with salt and pepper and set aside.
Combine the flour, cornstarch, black pepper, lemon pepper, Old Bay, garlic powder, Red Hot seasoning, and seasoned salt in a large bowl. Slowly whisk in the beer until the batter is light and about the consistency of wall paint. Refrigerate for 5–10 minutes.
In a separate bowl, combine the flour and cornstarch for the flour coating.
Add canola oil to a deep cast-iron skillet or deep fryer and heat to 375°F.
Pat the cod dry again. Coat each piece completely in the flour mixture, then dip it into the beer batter. Allow the excess batter to drip off for a few seconds.
Carefully lower the battered cod into the hot oil. Move it slightly when it first enters the oil to prevent sticking.
Fry in small batches for 3–4 minutes, or until golden and the cod reaches 145°F internally.
Toast the brioche buns. Place one slice of American cheese on the bottom bun, followed by the fried cod and tartar sauce. Add the top bun and serve immediately.