Preheat oven to 350 degrees F. Lightly coat eight 14-to 16-ounce individual baking dishes with cooking spray; set aside. In large skillet cook onion in butter over medium-low heat about 15 minutes or until tender and golden brown, stirring occasionally. Meanwhile, cook pasta in 4-quart Dutch oven according to package directions; drain and return to pan.
Add flour, salt, and pepper to onion in skillet; stir until combined, about 1 minute. 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 dishes.
Place bread pieces in food processor; cover and process to coarse crumbs. Sprinkle crumbs over pasta mixture. Bake, uncovered, 20 to 25 minutes or until heated through and crumbs are golden brown. If desired, add a wedge of Brie to each dish the last 5 minutes of baking time.