The night before, cook pasta according to package directions. Drain, rinse, and drain again. Place in a covered container and refrigerate overnight.
In a large skillet cook ground beef and onion over medium heat until meat is browned. Drain off fat; cool meat mixture slightly. Place meat mixture in the removable crockery liner of a 4- to 6-quart slow cooker. Stir in the next seven ingredients (through cumin). Cover and refrigerate overnight.
The next day, let the crockery liner stand at room temperature for 15 minutes. Place liner in the slow cooker unit. Cover and cook on low for 4 to 5 hours or on high for 2 to 2 1/2 hours. Stir cooked macaroni into meat mixture; let stand 15 minutes before serving. Top servings with tomato and/or green onions.