Lightly coat the inner surface of your slow cooker insert with nonstick cooking spray or softened butter to prevent sticking.
In a large mixing bowl, combine the cream of mushroom soup, sour cream, milk, garlic powder, salt, and pepper. Whisk until smooth. Fold in the chopped onion, optional mushrooms, half of the shredded cheese, and half of the crispy onions.
Add the prepared green beans to the greased slow cooker. Pour the sauce mixture over the beans, stirring gently to ensure even distribution.
Secure the lid and cook on the LOW setting for 4 to 5 hours, or on HIGH for 2 to 3 hours, until the beans have softened and the sauce is hot and bubbling.
Roughly 15 minutes before serving, scatter the remaining shredded cheese and reserved crispy onions across the surface of the casserole. Re-cover and allow to cook just until the cheese is fully melted.
Serve directly from the slow cooker while warm, spooning into individual portions.