Melt margarine or butter in a large saucepan over medium heat. Add celery and cook about 5 minutes or until tender.
Stir in condensed mushroom soup, water or broth, and onion soup mix. Heat until bubbly. Stir in chicken, heat through. Stir in rice' remove from heat.
To assemble, halve peppers lengthwise; remove stems, seeds, and membranes. Place pepper halves, cut sides up, in a large shallow baking or roasting pan. Divide chicken mixture among the pepper halves. Sprinkle with lemon-pepper seasoning.
Bake peppers, loosely covered with foil, in a 350 degree F oven about 45 minutes or until heated through. Top with cheese and pimiento. Bake, uncovered, about 5 minutes more or until cheese is just melted. Makes 10 servings.