Make Tartar Sauce: Combine mayonnaise, relish, capers, and lemon juice. Season with salt and pepper to taste. Set aside.
Prep the onions per instruction above.
Prep lettuce and tomato for the sandwich per instruction above.
Heat the oil in a heavy bottom pot to 350 degrees on medium heat.
Prep the batter: Combine flour, cornstarch, kosher salt, baking powder, paprika, and a few grinds of pepper in a large bowl then whisk to combine.
Batter and fry: Add beer or club soda to flour/cornstarch mixture and whisk until a batter is just formed. A few small lumps of dry flour are ok. Transfer fish to batter and turn to coat. Working one piece at a time, pick up the fish and allow excess batter to drip back into the bowl. Carefully lower it into the hot oil. Repeat with remaining fish.
Cook, turning the fish until it is golden brown and crisp on all sides, about 6-8 minutes to an internal temperature of 140°F (60°C) degrees using a thermometer. Transfer to a paper towel-lined plate and season immediately with salt.
Toss the onion rings in flour and then dip into the batter (working in small batches if necessary). Working one at a time, pick up the onion ring and allow excess batter to drip back into the bowl. Carefully lower it into the hot oil. Repeat with remaining rings until deep golden brown and crispy.
Lightly toast the bread in a toaster oven.
Assemble sandwich: Spread the tartar sauce on the inside of the bun. Layer with lettuce, fish, and tomato.
Serve: Enjoy sandwich with onion rings, ketchup, and beer.