Preheat oven to 350°. Cook pasta according to package directions for al dente.
Meanwhile, heat 3 teaspoons oil in a Dutch oven over medium-high heat. Add chicken in batches; cook and stir 3-5 minutes or until no longer pink. Remove from pan.
In same pan, add mushrooms to remaining oil; cook and stir over medium-high heat 3-5 minutes or until tender. Add garlic; cook 1 minute longer. Stir in soup, broth, 1 cup cheese, oregano and pepper. Add spinach and tomatoes; return chicken to pan.
Drain pasta; add to soup mixture and toss to combine. Divide between 2 greased 8-in. square baking dishes. Sprinkle with remaining cheese. Bake, covered, 15-20 minutes or until heated through. Freeze option: Cool unbaked casseroles; cover and freeze. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Cover casseroles with foil; bake as directed, increasing baking time to 1-1/4 hours or until heated through and a thermometer inserted in center reads 165°.