Sauté onion & bell pepper in margarine. Combine cheese, milk, and soup in a pan. Cook over low heat until cheese is melted. Add onion & black pepper. Combine thawed broccoli, chicken, rice & salt with cheese mixture. Pour into buttered casserole dish. Bake at 350° F for 30 minutes. Casserole is done when mixture thickens & bubbles around the edges.