Preheat oven to 375° F.
Combine tuna, onion, celery, lemon juice, and 2 tablespoons mayonnaise in a medium bowl; set aside.
Combine remaining 2 tablespoons of mayonnaise and ketchup in a small bowl; set aside.
Slice bun horizontally into three equal slices.
Spread bottom layer with mayonnaise-ketchup mixture, then top with sweet pickle slices.
Make a double decker with middle bun slice and tuna.
Repeat for remaining sandwiches.
Wrap in aluminum foil and bake for 15 minutes.
Serve hot.
