Preheat oven to 350°F Lightly coat baking dish with cooking spray.
In large skillet, cook onion in butter over medium-low heat about 15 minute or until tender and golden brown.
Meanwhile, cook pasta according to package; drain and return to pan.
Add flour, salt and pepper to onion in skillet. Stir. Add milk all at once. Cook and stir until slightly thickened and bubbly. Gradually add chopped cheese; cook over medium-low heat until cheese melts. Stir into pasta. Fold in crab. Transfer to baking dish. Sprinkle with crumbs.
Bake 20-25 minute or until heated through and crumbs are golden brown.