In 3 1/2- to 6-quart slow cooker, combine gravy mix and broth; stir until smooth. Add chicken, stew vegetables and mushrooms; mix well.
Cover; cook on low setting for 4 hours.
About 1 hour 10 minutes before serving, blend sour cream and 1 tablespoon flour until smooth. Stir into chicken mixture. Increase heat setting to high; cover and cook 20 minutes.
Combine 1 1/2 cups flour, onions, baking powder and salt. With pastry blender or fork, cut in shortening until mixture resembles fine crumbs. Stir in milk just until moistened. Stir in peas. Drop dough by rounded tablespoonfuls onto chicken mixture. Cover; cook on high setting 45 to 50 minutes or until toothpick inserted in center of topping comes out clean.