Combine tuna, onion, celery, juice, & 2 tbsp. mayo in medium bowl. Combine remaining mayo & ketchup in small bowl. Slice bun horizontally into 3 equal slices. Spread bottom layer w/ mayo-ketchup mixture, then top w/ sweet pickle slices. Make double decker w/ middle bun slice & tuna. Repeat for remaining sandwiches. Wrap in foil & bake at 375F for 15 minutes .Serve hot.