HEAT oil in large nonstick skillet on medium-high heat. Add mushrooms and onions; cook 5 min., stirring occasionally. Stir in tomatoes and dressing.
ADD chicken; cover. Simmer on medium-low heat 12 min. or until chicken is done (165ºF). Meanwhile, cook spaghetti as directed on package, omitting salt.
TOP chicken with cheese and bacon; simmer, covered, 5 min. or until cheese is melted. Drain spaghetti; place on platter. Top with chicken mixture.