2 pounds red potatoes, peeled and thinly sliced (about 4 cups)
1 cup thinly sliced onions, divided
Preheat oven to 350°. In a small saucepan, melt butter; stir in flour, salt and pepper until smooth. Gradually whisk in milk and Hidden Valley® Original Ranch® Seasoning & Salad Dressing Mix Shaker. Bring to a boil, stirring constantly; cook and stir until thickened, about 2 minutes. Remove from heat; stir in cheese until melted.
Coat an 8-inch square baking dish with cooking spray. Place half of the potatoes in dish; layer with 1/2 cup onion and half of the cheese sauce. Repeat layers.
Bake, covered, 50 minutes. Uncover; bake until bubbly and potatoes are tender, 10-15 minutes longer.