Preheat oven to 400°. Cut squash, zucchini and peppers into 2 x 1 1/2-inch strips. Slice red onion. Cut mushrooms in quarters if large. Place all vegetables in large roasting pan or casserole. Mix together oil, thyme and pepper. Pour over vegetables in pan and mix well. Cook 20 minutes, until tender-crisp, stirring occasionally. Add salt to taste.