Spray a 3 quart casserole dish or a 9x13 dish with cooking oil and set aside.

Wash and peel the potatoes, then cut them in half and add them to a large pot of salted water. Bring the potatoes to a boil and cook for 8-10 minutes. They should be nearly tender, but still have some bite to them. Let the potatoes cool until you are able to handle them, then using a box grater shred them using the largest grate.

In a large bowl whisk together the melted butter, cream cheese, Greek yogurt, milk, salt, and pepper. Add in the shredded potatoes, green onions, and 1/2 a cup each of the shredded mozzarella and cheddar cheese. Taste for seasoning and add more salt if needed.

Fold everything together until combined then pour the potatoes into the prepared casserole dish. Top with the remaining cheese. Bake the potatoes at 375° F. for about 30 minutes or until the cheese is melted and bubbly.