Heat oil in a large nonstick skillet over medium-high heat. Add onion; sauté 3 minutes or until tender. Add chicken; sauté 30 seconds or until thoroughly heated. Remove from heat; stir in cheese and salsa.
Warm tortillas according to package directions. Spoon about 1/2 cup chicken mixture down center of each tortilla; roll up. Wipe pan clean with a paper towel. Heat pan over medium-high heat. Coat pan with cooking spray. Place filled tortillas in pan, seam sides down; cook 4 minutes, turning to brown on all sides.