Place spinach in a strainer and set aside to drain well. Prepare pasta according to directions.
While pasta cooks, place oil in bottom of a 7x11 inch baking dish (with cover). Add onion in a single layer and bake, uncovered, about 15 minutes or just until tender. Transfer onions to a large bowl and set baking dish aside.
Press spinach between 2 paper towels to remove any remaining liquid. Stir pasta, spinach, chicken, tomatoes, cream cheese, salt and pepper into onion in bowl. Spoon into baking dish and top with cheese.
Bake, covered, for 30 minutes; uncover and bake 15 more minutes or until bubbly.
Recipe excerpted from Alabama Hometown Cookbook, copyright 2016, and reprinted with permission.