In a large skillet, brown sausage and cayenne over medium heat until meat is no longer pink, 5-7 minutes, breaking into crumbles; drain.
Transfer to a 1-1/2- or 2-qt. slow cooker. Add onion and green pepper. Pour spaghetti sauce on top. Cover and cook on high for 1 hour. Reduce heat to low and cook 2-3 hours longer. Serve over spaghetti.