Rinse filets and pat dry with paper towels; cut into sizes appropriate for sandwiches.
In a flat dish, beat the liquid of your choice and egg until well blended. In another flat dish, combine flour, cracker or bread crumbs, baking powder, salt, pepper, and paprika until smooth.
Dip filets into the egg mixture, then into the breading mixture (pat breading onto the filets with your fingers), shaking off excess. Place the breaded filets in a single layer on a platter or pan; refrigerate for 30 minutes (this will allow the breading to set).
Preheat oven to 150 degrees F. In a large frying pan, heat 1/2-inch of vegetable oil to 365 degrees F. Add the breaded filets and fry 3 to 5 minutes on each side, turning once, or until fish flakes easily with a fork and is golden brown. Remove from hot oil and drain on paper towels. Keep war, uncovered in the oven.
Serve on hamburger buns with lettuce leaves, sliced tomatoes, and tartar sauce.
Makes 4 sandwiches.