Peel onions; cut flat slice off base of each onion so they will stand upright. Hollow them out, leaving 1/2-inch shells. Chop onion from centers. Place onion shells in large skillet, add 1-inch water, cover and bring to boil. Simmer gently for 25 minutes or until soft. Remove from skillet with slotted spoon and drain. Empty skillet; add oil and heat. Add chopped onion, squash, mushrooms, celery, red pepper and garlic; saut over medium heat until tender-crisp, about 6 to 8 minutes. Add tomatoes, parsley, tarragon and wine and cook 2 minutes longer. Nestle onion shells into pan and fill with vegetable mixture. Makes 4 servings.
*Chicken or vegetable broth may be substituted.