SERVINGS 8 PREP TIME 40 mins Ingredients   Nonstick cooking spray 1 pound dried medium shell macaroni 5 tablespoons butter 1 medium sweet onion, halved and thinly sliced 1/3 cup all-purpose flour 3/4 teaspoon salt 1/2 teaspoon ground black pepper 3 cups milk 1 pound Brie cheese, rind removed and cheese coarsely chopped (reserve 8…