Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the mashed potatoes, sour cream, and milk.
Stir in melted butter, garlic powder, salt, and pepper until well blended.
Fold in shredded cheddar cheese and green onions.
Transfer the potato mixture to a greased 9x13-inch baking dish.
Spread the mixture evenly and smooth the top with a spatula.
Sprinkle breadcrumbs over the surface for a crunchy topping.
Bake in the preheated oven for 25-30 minutes until golden and bubbly.
Let it cool for 5 minutes before serving for better slicing.
Garnish with extra green onions if desired.