Place the beef in a 2-quart casserole. Cover with lid, and microwave at HIGH 8 minutes, stirring after 4 minutes. Drain well. Combine beef, salsa, rub, and beans in dish. Cover; microwave at HIGH 4 minutes or until hot.
Place the polenta in a 9-inch pie plate coated with cooking spray. Spoon beef mixture on top of polenta, and cover with plastic wrap. Microwave at high 6 minutes or until thoroughly heated. Uncover; sprinkle with cheese.