In large skillet, cook ground beef over medium heat until thoroughly cooked, stirring frequently. Drain. Add half of the onions, soup, milk and pepper; mix well.
Spray 3 1/2 to 4-quart slow cooker with nonstick cooking spray. Layer potatoes and green beans in sprayed slow cooker. Top with beef mixture; spread evenly.
Cover; cook on Low setting 6 to 7 hours. Top with remaining half of onions before serving.