Preheat oven to 350 degrees F. Spray large nonstick skillet with cooking spray. Add onion; cook over medium-high heat for 4 to 5 minutes or until tender. Add chicken, 1/2 cup cheddar cheese, 3/4 cup salsa, and cream cheese. Heat and stir for 2 to 3 minutes or until cheeses are melted.
Spoon a scant 1/3 cup chicken mixture down center of each tortilla; roll up. Place in 13x9-inch (3-quart rectangular) baking dish; drizzle with remaining 1-1/4 cups salsa. Sprinkle with remaining cheddar cheese; cover. Bake for 15 minutes or until heated through and cheese on top is melted.