Heat oven to 350 degrees. Spray a 13 x 9 casserole dish with cooking spray. Cook pasta as directed and drain. In a large skillet, cook onion in butter, over medium heat, until onion is tender. Add flour, salt and pepper to onion. Stir until combined, about 1 minute. Stir in the milk, then cook, stirring, until slightly thickened. Gradually add the cheese and cook over medium low until cheese melts. Stir ingredients into pasta. Fold in the crab meat and transfer to casserole dish. Sprinkle with breadcrumbs. Bake, uncovered, for 25 minutes or until golden brown.