In a large saucepan, cook sausage and onion in boiling water for 2 minutes; drain. In a large bowl, combine the ketchup, sugars, vinegar, molasses and mustard. Stir in beans and sausage mixture.
Transfer to a greased 2-1/2-qt. baking dish. Cover and bake at 350° for 1-1/2 hours or until bean mixture reaches desired thickness.
Editor's Note: This recipe can also be prepared in a slow cooker. Prepare as directed, transferring to a slow cooker instead of a baking dish. Cover and cook on low for 6-8 hours or until heated through.