Mix the tuna, onions, mayonnaise, and vinegar in a bowl. Flavor with salt and pepper.

Cut bread open and toast. Put one teaspoon of mustard on one side of each bread.

Split the tuna mixture between the 2 baguettes and spread it over the mustard. Cover with tomatoes and add cheese on top of it.

Close the sandwiches and place them in a preheated oven until the cheese melts (10 minutes).

To serve, cut in half each sandwich and place on a plate.