Preheat oven to 375°F (190°C).
Sauté vegetables: In a skillet, sauté onion and celery until onions are translucent. Add mushrooms and cook for a few more minutes, then add garlic and cook for one minute or until garlic is fragrant.
Make the stuffing: In a medium saucepan, bring chicken broth and butter to a boil. Add stuffing mix and stir until evenly moistened. Remove from heat and cover. Let stand for 5 minutes.
Combine filling ingredients: In a large bowl, combine shredded chicken, cream of chicken soup, sour cream, sautéed vegetables, Worcestershire sauce, seasonings, and frozen veggies. Stir well.
Assemble casserole: Spread the chicken mixture in an 8x8 casserole dish. Top with stuffing.
Bake: Bake for 40 minutes, or until heated through and stuffing is lightly browned.