Preheat oven to 425 F. Lightly coat a baking sheet with cooking spray.
Cut each chicken breast into cubes. In a large, nonstick frying pan, add the chicken and onions. Saute until the onions are tender and the chicken is cooked through, about 5 to 7 minutes. Remove from heat. Stir in the salsa, tomatoes and cilantro.
Lay a tortilla flat and rub the outside edge with water. Spread about 1/2 cup of the chicken mixture onto the tortilla, leaving about 1/2 inch free around the outer rim. Sprinkle with a spoonful of shredded cheese. Fold tortilla in half and press to seal. Place on a cookie sheet. Repeat with the rest of the tortillas.
Lightly coat the top of the tortillas with cooking spray. Bake until the quesadillas are lightly browned and crispy, about 5 to 7 minutes. Cut in half and serve right away.