Add the warm milk, butter, sour cream, cream cheese, and mash with a potato masher. Add half of the cheddar cheese, green onions, garlic powder, salt, and pepper. Stir to combine, then taste and adjust the salt and pepper as needed or add more milk if the potato filling seems too dry.
1/4 cup whole milk, 1/4 cup salted butter, 1/4 cup sour cream, 2 ounces cream cheese, 1 1/2 cups shredded cheddar cheese, 3-4 green onions, 1/2 teaspoon garlic powder, 1/2 teaspoon coarse kosher salt, 1/2 teaspoon black pepper