Meanwhile, heat margarine or butter in a 12-inch skillet. Add chicken and mushrooms, and cook about 5 minutes or until chicken is no longer pink. Remove from skillet.
Shake together chicken broth, oregano, flour, garlic salt, and pepper in a screw-top jar until smooth. Add to skillet along with milk. Cook and stir until thickened and bubbly; add cream cheese. Cook and stir until cream cheese is smooth. Remove from heat. Stir in lemon juice. Add pasta mixture, chicken mixture, tomatoes, and sweet pepper. Toss to coat. Turn into an ungreased 13" x 9" x 2" baking dish. Bake, covered, in a 350 degrees F. oven for 30 to 35 minutes or until heated through. Stir mixture and sprinkle with Parmesan cheese.