{{model.addEditText}} Print. 1  Prep. 2  Cook. 3  Ready In.  Preheat oven to 350 degrees F (175 degrees 1  C). In a mixing bowl, combine sweet onion, drained tuna, mozzarella, and mayonnaise.  Spread tuna mixture on slices of French bread to form a 1  sandwich. Bake in a preheated oven for 10 minutes.