Place oil in a large frying pan.

Place pan over medium heat.

When oil is hot and shimmering, add chicken. Sauté until chicken pieces are browned and cook through.

Remove chicken to a paper towel-covered plate to drain.

Place pan back on heat.

Add water and fajita seasoning mix to the pan. Stir to mix. Cook until sauce has thickened slightly, about 2 minutes. Remove from heat. Combine chicken and sauce. Set aside.

Lay tortillas on a work surface (a few at a time, if you are pressed for space).

Butter one side of all the tortillas. Set aside.

In a small bowl, add diced tomato and onion, Mix. Set aside.

Place a clean frying pan or skillet over medium heat - Or heat up a griddle.

When surface is hot, place a tortilla, buttered side down, on hot pan or griddle.

Layer about 1 tablespoon each of the ingredients in the following order on the tortilla - cheese, chicken and sauce, tomato/onion mixture, more cheese, salsa - spreading evenly over all of the tortilla.

Top with another tortilla, buttered side up.

Once the cheese starts to bubble from the sides of the quesadilla and shell is crunchy and a golden color, very carefully flip it over to cook the other side.

When other side is also crunchy and golden, remove from heat to a plate or cutting board.

Repeat with remaining tortillas and fillings.

Let quesadillas stand for a few moments, then cut into quarters.

Serve with sour cream, guacamole or salsa for dipping.