Preheat oven to 350ºF. In a skillet, heat margarine until bubbly. Add onion, sauté until soft. Add zucchini, mushrooms, celery, parsley, and thyme. Sauté until vegetables are tender-crisp, about 5 minutes. Remove from heat and add rest of ingredients. Spray 3-quart casserole; spoon vegetable mixture into casserole. Cover; bake 15 minutes. Remove cover; bake an additional 5 minutes.