Cook spaghetti according to package directions. Set aside.

Sauté onion in two tablespoons oil in skillet until golden. Place cubes of chicken in a 9×13-inch pan with the next three vegetables and the spaghetti. Combine sauce ingredients and pour into pan, mixing well to coat.

Bake, covered, stirring every 20 minutes, at 350 degrees Fahrenheit, for one hour or until the chicken is cooked through.