In a 2 quart casserole combine the lemon peel, juice, tarragon, and salt. Add onion. Cook, covered on 100% power (high) for 3 minutes. Add mushrooms, squash, celery, and pimiento, toss gently to coat with lemon mixture. Cook, covered, on high for 6 to 9 minutes (10 to 12 minutes in low wattage ovens) or until the vegetables are tender, stirring often. Serve with a slotted spoon.