Slice the onions and add them to a skillet with oil and a small cup of water. Cook over medium-high heat, adding a pinch of salt and stirring occasionally until the onions change color. Once the onions are golden, add the bell peppers sliced into strips and continue cooking the same way. Add another splash of oil and a good pinch of salt (and a little water if the vegetables start to stick to the pan).

Meanwhile, prepare the chicken tenderloins. Cut them into bite-sized pieces and season with the fajita seasoning packet. Mix well with your hands to coat evenly, then set aside.

When the peppers are cooked to your liking (I like them very well done, so I cooked them for another 15 minutes), add the seasoned chicken. Stir occasionally until all the chicken pieces are cooked through and no longer pink.

Once the chicken is fully cooked (you can taste a couple of pieces to be sure), check for salt and adjust to taste. Then start assembling the quesadillas! On one half of a flour tortilla, spread a tablespoon of tomato sauce and add some cubes of cheese.

Top with the chicken and vegetable mixture, then add more cheese cubes. Fold the tortilla in half to close the quesadilla.

In a skillet over medium heat, add a splash of oil and toast the quesadilla for about 1 minute on each side, until golden brown.

And they're ready! If you're really hungry, you might eat two without a problem. Otherwise, I recommend one per person with some nachos to share.