Preheat oven to 350℉. Spray an oven safe baking dish with non-stick cooking spray. Set aside.
Fill a large pot with water and bring to a boil over high heat.
Peel and slice potatoes into ¼ inch thick slices using a mandolin or sharp knife to slice the potatoes.
Place slices in a bowl with water to prevent them from browning while you finish peeling and slicing the remaining potatoes.
Drain off water in the bowl and boil potato slices for 4 minutes, or just until tender. Drain and set aside while you make the sauce.
Melt butter in a large saucepan over medium heat. Add cream stirring until it's warm, about 2-3 minutes.
Add half the shredded cheddar cheese to the pan and stir until the cheese is completely melted.
Remove pan from the heat and stir in sour cream.
Add ranch seasoning mix, green onions, salt and pepper to the pan. Stir until you have a thick, smooth, creamy cheese sauce.
Layer half the sliced potatoes in the prepared baking dish.
Top potatoes with half the cheese sauce.
Top with a layer using the remaining potatoes and finish with a layer using the remaining cheese sauce.
Top everything with a layer of the remaining shredded cheddar cheese.
Cover the pan with foil and bake for 45 minutes or until heated through and cheese is hot and bubbly.
Remove foil and turn oven temperature to broil. Toast until cheese is golden bubbly brown along the edges. Keep a close eye on it so it doesn't burn.
Remove from oven. Allow casserole to rest for 10 to 15 minutes before serving to allow it to set and firm up.