Preheat oven to 350 degrees F. Grease the inside of a 9-inch x 13-inch baking dish. Set aside.
Thinly slice potatoes, about 1/4-inch thick. You can use a chef's knife or a mandolin for more even slices. Layer the potatoes in your prepared baking dish.
In a large bowl, stir together soup, onion soup mix, and milk until combined. Pour over potatoes.
Tightly cover baking dish with foil and bake for 50 minutes.
Remove foil from dish and sprinkle top of potatoes with cheddar cheese. Return to oven for 5 minutes, until cheese is melted and bubbly.
Remove baking dish from oven and sprinkle crispy onions all over the top. Serve immediately.