Preheat oven to 375 degrees.
Grease the inside of a 2 quart baking dish with cooking spray or butter.
Slice potatoes into ¼" thick rounds and layer flat in the baking dish.
In saucepan on medium-low heat melt butter and add flour to form a roux. Whisk continually for 5 minutes to prevent roux from burning.
When the roux begins to deepen in color add buttermilk slowly, mixing to combine as you pour.
Add ranch seasoning and black pepper to taste.
Slowly add ¾ cups shredded cheddar cheese in batches, ensuring each batch fully melts before more is added.
Continue to cook until the mixture thickens slightly, taste and add salt only if needed.
Pour sauce over potatoes in the baking dish, cover with foil and place in the oven for 1 hour.
Uncover potatoes, add remaining ¾ cup of cheese and bake 25-30 more minutes. Cheese should be nicely browned.
Remove from oven and let cool for 5 minutes before serving.