Grease a 6-quart slow cooker.
Add drained green beans, cream of mushroom soup, milk, Worcestershire sauce, Parmesan cheese, salt, and pepper.
Stir until green beans are coated.
Cover and cook on Low for 2 hours.
Sprinkle crispy fried onions on top.
Cover and cook 30 minutes more.
Serve warm from slow cooker.