Preheat an oven to 375F and grease a 9x13 casserole dish with a little butter or olive oil spray.
Shred your cheeses and mix them together. Set aside half of the cheese for the filling and the other half to top the casserole.
In a large bowl, add the thawed potatoes, melted butter, seasonings, mustard, green onion and sour cream. Mix well.
Then, add the half-and-half and shredded cheese you set aside for the filling. Mix well. Give the potatoes a taste to see if more seasoning is needed.
Then, transfer the potatoes to the prepared casserole dish and add the remaining shredded cheese to the top.
Next, place the casserole dish into the preheated oven and bake for about 20-25 minutes, or until it’s bubbly and golden on top. Feel free to broil to get the desired color you want to achieve. Then, let the potatoes cool for about 5 minutes before serving.
Garnish with more chopped green onion and enjoy!