Heat butter in large skillet over medium heat; add onion, celery, mushrooms and carrots and cook 8-10 minutes, or until tender. Sprinkle with flour, stir and cook 30 seconds. Remove from heat; stir in stock. Slowly add milk, stirring constantly. Return to heat and bring to a simmer.
Mix veggies, chicken, rice, Italian seasoning, garlic powder and pepper in large mixing bowl.
Transfer mixture to 8x8-inch baking dish. Bake, uncovered, 25-30 minutes or until heated through.