Preheat oven to 375 degrees.
Spray a 9" x 13" baking dish with nonstick spray.
Stir Ranch Seasoning with milk. Set aside.
Wash potatoes and thinly slice into 1/4" thickness. Set aside.
Dice onion. Set aside.
Assemble: Place 1/2 of sliced potatoes in the bottom of the baking dish, slightly overlapping. Sprinkle 1/2 of the diced onions on top of the potatoes. Add 1 cup of shredded cheese to the top of the onions. Pour 1/2 cup milk/ranch over the top.
Repeat layers. Potatoes, onions, cheese, and milk/ranch seasoning.
Top with Cream of Mushroom Soup. Spreading over the top.
Cover with foil and bake at 375 degrees for 1 hour.
Uncover and let sit 5-10 minutes before serving.
