Prepare the fish: Cut the fish fillets into portions and season with salt and pepper. Dredge each piece in flour, dip in the beaten egg, and then coat in breadcrumbs.
Fry the fish: Heat vegetable oil in a large skillet over medium heat. Fry each fillet for 3-4 minutes on each side until golden and crisp. Drain on paper towels to remove excess oil.
Make the tartar sauce: In a small bowl, mix the mayonnaise, pickles, lemon juice, and mustard. Season with salt and pepper to taste.
Assemble the sandwich: Lightly toast the buns. On the bottom half, place a fish fillet, spoon on tartar sauce, add a slice of cheddar cheese (optional), and a lettuce leaf. Top with the other half of the bun.
Serve: Enjoy your homemade Fish-O-Fillet sandwich while the fish is still warm and crispy.