1. Preheat oven to 375 degrees.
2. Combine tuna, onion, celery, lemon juice, and 2 tablespoons mayonnaise in a small bowl; set aside.
3. Combine remaining 2 tablespoons of mayonnaise in a small bowl; set aside.
4. Slice bun horizontally into 3 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.
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