Cut chicken into bite-size pieces; cook chicken and mushrooms in olive oil in a large skillet until chicken is no longer pink.
Remove from skillet, set aside.
Cook tortellini according to package directions, adding the frozen vegetables during the last full minute of cooking time. Drain and rinse in cold water.
Combine chicken broth, flour, oregano, garlic salt, and black pepper (this can be done in a screw-top jar, shake until smooth) add to skillet along with the milk.
Cook and stir until thick and bubbly; add cream cheese.
Cook and stir until cream cheese is melted and mixture is smooth.
Remove from heat, stir in lemon juice. Add pasta mixture, chicken mixture, and tomatoes.
Toss to combine.
Spoon into a 3-quart baking dish.
Bake covered, at 350° for about 30 minutes or until heated through. Stir and sprinkle with parmesan cheese.
This dish can be made 24 hours ahead. Increase baking time to 55-60 minutes.