In 10-inch skillet, cook beef over medium-high heat 5 to 7 minutes, stirring occasionally, until brown; drain. Stir in soup, milk, pepper and half of the onions.
Spray 3 1/2- to 4-quart slow cooker with cooking spray. Layer potatoes and green beans in cooker. Top with beef mixture; spread evenly.
Cover; cook on Low heat setting 6 to 7 hours. Top with remaining onions before serving.