Preheat your oven to 400°F (200°C).
Wash the russet potatoes and pierce them with a fork. Bake for about 45-60 minutes until tender.
Allow the potatoes to cool slightly, then peel and mash them in a large bowl.
Add sour cream, milk, butter, half of the cheese, beef, green onions, salt, and pepper to the mashed potatoes.
Mix everything until well combined.
Transfer the mixture to a greased casserole dish and top with the remaining cheese.
Bake in the oven at 350°F (175°C) for 30 minutes or until hot and bubbly.