Heat oven to 450°. In a medium-size bowl, stir together chicken, cheese, onion and taco seasoning. Stir in cilantro, if desired.
Place 2 tortillas on baking rack over a baking sheet. Spread 1 heaping cup of chicken mixture over each tortilla. Top each with another tortilla and press down gently. Bake quesadillas at 450° for 8 minutes or until cheese has melted. Cut into quarters and serve with salsa, if desired.