Preheat oven to 350. Prepare a 2-quart casserole dish with cooking spray; set aside. Cook ravioli according to package directions; drain and set aside. Meanwhile, in a large skillet, cook chicken and onions until chicken is no longer pink. Add spaghetti sauce, spinach, and salt. Mix well. Spread half the chicken mixture into prepared dish. Arrange ravioli over chicken layer. Spread remaining chicken mixture over ravioli. Bake, covered, for 20 minutes. Uncover, and sprinkle cheese over top. Bake for five minutes more, or until cheese has melted.