1. Cook onion and celery in margarine in medium-size skillet, covered, over medium heat until tender, 8-10 minutes. Scrape into bowl. 2. Soak matzohs in water to cover 3-5 minutes. Drain; squeeze dry. Add to bowl. Add chestnuts, mushrooms, thyme, rosemary, sage, parsley, salt, pepper and eggs. Stuff 12-14 pound turkey; roast as you ordinarily would. Or bake stuffing in greased shallow 2-quart oven-proof casserole in preheated moderate oven (350'F) for 45 minutes.