Preheat oven to 400 degrees F. Spray an 11 x 7-inch baking dish with cooking spray and set aside.
In a deep skillet or sauce pan combine the butter over medium heat, add the shallot and garlic and sauté until fragarant. Stir in the milk, heavy cream, cheeses, and Ranch seasoning.
Bring the mixture to a simmer, stirring occasionally. When the mixture has thickened remove from the heat and set aside.
Layer the thinly sliced potatoes in a greased casserole dish. Pour the cheddar sauce over the potatoes and tilt back and forth so it is evenly distributed. Cover the dish with foil and bake 25 minutes.
Remove the foil and add the remaining cheese. Bake an additional 10-15 minutes until the potatoes are fork tender and the cheese is golden.
Allow the dish to rest at least 10 minutes before serving.
Notes
Cooking time may vary slightly based on the thickness of your potatoes. To check potatoes, stick a fork or knife for tenderness.