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