In a large skillet heat the 1 tablespoon oil over medium heat. Add chicken; cook for 12 to 15 minutes or until chicken is no longer pink (170 degrees F), turning once. Remove chicken from skillet.
Add onion and garlic to skillet. Cook for 3 to 5 minutes or until onion is tender, stirring occasionally. Add tomato; cook and stir for 2 to 3 minutes or until tomato is softened. Return chicken to skillet; heat through. Remove from heat; cover and keep warm.
In an extra-large skillet heat the 1 teaspoon oil over medium heat. Add mushrooms; cook for 2 to 3 minutes or just until tender, stirring occasionally. Add spinach all at once; cook for 1 to 2 minutes or just until wilted, tossing with tongs.
Divide spinach mixture among dinner plates; top with chicken and tomato mixture. Sprinkle with cheese and basil.