In a mixing bowl, combine Sweet Onion, drained Tuna, Mozzarella, and mayonnaise, mix thoroughly.
Cut French bread or hoagie bun length wise in half, foil cookie sheet, spray Pam on foil, spread tuna mixture on slices of bread for open faced sandwich.
Bake in a preheated oven for 10 minutes, or till bread edges just start to brown and cheese and tuna mixture start to bubble. Enjoy.
