Preheat oven to 375°F and grease a 9×13 baking dish.
Brown sliced sausage 5–7 minutes until lightly crisp.
Sauté onion 4 minutes, add garlic and cook 1 minute.
Mix potatoes, sausage, onion, garlic, cheese, ranch mix, milk, butter, salt, and pepper.
Transfer to baking dish and cover tightly with foil.
Bake covered 60 minutes until potatoes are tender.
Remove foil and bake 15 minutes until golden, broil 2 minutes if desired.
Rest 5 minutes, garnish with green onions and serve.