Prepare the avocado salad. Dice the avocado into bite-sized pieces. Add to a medium bowl with the chopped, seeded tomato and sliced red onion. Add the lemon juice and drizzle with olive oil. Sprinkle with salt and pepper. Set aside while preparing the quesadillas.

Preheat the oven to 425F. Lightly coat a baking sheet with cooking spray.

In a large, nonstick frying pan, add the chicken and onions and sauté until the onions are tender and the chicken is thoroughly cooked, about 5 to 7 minutes. Remove from heat. Stir in the salsa, tomatoes and cilantro.

Cut each chicken breast into bite-sized cubes.

To assemble: lay a tortilla flat and 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. Place on the prepared baking sheet. Repeat with the remaining tortillas. Lightly coat the top of the tortillas with cooking spray.

Bake until quesadillas are lightly browned and crispy, about 5 to 7 minutes. Place the quesadillas under the broiler during the last minute or so of cooking for a more crisp finish. Cut in half and serve immediately.