Heat oven to 350°F. Lightly grease a 2-quart baking dish.
Whisk soup, milk, soy sauce, pepper, salt, and garlic powder in a large bowl.
Fold in green beans, 1 cup fried onions, and the cheddar if using.
Spread mixture in the dish. Cover and bake 20 minutes until hot and bubbling at edges.
Stir, then top with remaining onions. Drizzle melted butter over the top if using.
Bake uncovered 8–10 minutes until the topping is deeply golden. Rest 5 minutes before serving.