Add the broccoli cheddar soup mix, heavy cream, and stock to a large bowl. Mix and set aside for at least 10 minutes.
Once soup has hydrated in the liquids, mix together in a large bowl with the thawed potatoes, sour cream, half the cheese, salt and black pepper.
Pour melted butter into a large 9x13 baking / casserole dish. Spread around to coat all sides. Transfer potato mixture to the casserole dish and then sprinkle the top with remaining cheese.
Bake in a 350F preheated oven for 45 - 50 minutes. Allow to rest for 10 minutes before slicing and serving.