Heat oven to 350. Spray a 9 x 13" casserole dish w/ cooking spray. In lg skillet, cook onion in butter over mediume heat until onion is browned and tender. Cook pasta as directed, drain. Add flour, salt and pepper to skillet w/ onions, stir until combined, 1 min. Stir in milk. Cook, stirring until slightly thickened. Gradually add cheese chunks. Cook over medium-low until cheese melts. Stir ingredients into pasta. Fold in crab. Transfer to casserole dish. Sprinkle w/ crumbs. Bake, uncovered 20-25 mins or until golden brown.