Prepare the Fish. Pat the fish fillets dry with paper towels. Dry fish helps the coating stick better and fry up crispier. If the fillets are very large, cut them into sandwich-sized portions.
Set Up the Breading Station. In one shallow bowl, mix the flour, cornmeal, garlic powder, paprika, salt, and black pepper. In another bowl, whisk together the egg and buttermilk.
Bread the Fish. Dip each piece of fish into the buttermilk mixture, then dredge it in the seasoned flour and cornmeal mixture. Press gently so the coating sticks well on all sides.
Fry Until Crispy. Heat oil in a deep skillet or heavy pan over medium-high heat. Once the oil is hot, carefully add the fish and fry for 3 to 5 minutes per side, depending on thickness, until golden brown and cooked through. Transfer to a paper towel-lined plate.
Assemble the Sandwich. Spread tartar sauce or mayonnaise onto the buns. Add lettuce and pickles, then place the crispy fried fish on the bun. Top with the other half of the bun and serve immediately.