In large bowl, combine broccoli, celery soup, 1C. cheese and onions. Pour into lightly greased Crock-Pot. Sprinkel top with crushed crackers, then with remaining cheese. Cover and cook on Low 5 to 6 hours or on High 2 1/2 to 3 hours.
NOTE:If desired, casserole may be spooned into a backing dish and garnished with additional grated cheese and crushed potato chips; bake 5 to 10 minutes in an oven at 400 degree F.