1Heat oven to 450ºF. Toss asparagus, bell pepper, onion, mushrooms, oil, lemon-pepper seasoning and salt until vegetables are coated. Spread in ungreased jelly roll pan, 15 1/2x10 1/2x1 inch. Roast about 20 minutes or until vegetables are crisp-tender. Cool slightly. Coarsely chop vegetables.
2While vegetables are roasting, cook and drain manicotti as directed on package. In 1 1/2-quart saucepan, mix sauce mix and milk. Heat to boiling, stirring constantly. Stir in marjoram; remove from heat.
3Reserve 1 cup vegetables for topping. Mix remaining vegetables, 1 cup of the cheese, the chicken and 1/2 cup of the sauce. In bottom of ungreased rectangular baking dish, 13x9x2 inches, spread about 1/4 cup sauce. Spoon chicken mixture into manicotti shells; arrange in dish. Spoon remaining sauce over manicotti. Sprinkle with remaining 1 cup vegetables and 1/2 cup cheese.
4Cover and bake 30 minutes. Uncover and bake about 10 minutes or until bubbly.