For the filling:
Cook onions, red bell pepper, and tuna in a hot skillet until soft, add salt and pepper, once cook, place in a bowl and add half of the cilantro and the ketchup.
For the quesadillas:
In the same skillet add more oil, place one tortilla, cheese, some of the filling more cheese and another tortilla, cook for about 3 minutes on each side, or until golden brown.
Place on the board cut in fourths, place in the container or serve immediately. These quesadillas are great at room temperature also.