Heat oven to 350°F. Grease a 2-quart baking dish with a little butter.
Whisk soup, milk, soy sauce, garlic powder, salt, and pepper in a large bowl until smooth.
Fold in the green beans, about 3/4 cup fried onions, and the cheddar if using.
Spread the mixture into the prepared dish and dot the top with any remaining butter.
Bake for 25 minutes until hot and bubbling around the edges.
Top with the remaining fried onions and bake 8–10 minutes more, until crisp and golden.
Let rest 5 minutes before serving to set the sauce.