Spray the base of your slow cooker with non-stick spray.
Trim the ends of your beans and cut them in half or thirds (depending on the length). Then, add them to your slow cooker along with 1 ½ cups of the French-fried onions, 3 cans cream of mushroom soup, whole milk, onion, salt, and ground pepper. Mix well to combine.
Cook on HIGH for 4 hours or until desired tenderness of beans. Top with the remaining French-fried onions and serve.