Drain chopped spinach well, pressing between paper towels to remove excess moisture.
Sauté squash, bell pepper, and onion in hot oil in a large skillet over medium-high heat 6 minutes or until tender. Remove from heat. Stir in spinach, chicken, next 6 ingredients, and 1 1/2 cups cheese. Spoon into a lightly greased 13- x9-inch baking dish.
Bake at 350° for 30 minutes. Sprinkle evenly with remaining 1/2 cup cheese, and bake 5 more minutes.
*Reduced-sodium, reduced-fat cream of celery soup, light sour cream, and reduced-fat sharp cheddar cheese may be substituted.