1. Preheat the oven to 350°F and lightly grease a medium casserole dish.
2. In a mixing bowl, stir together the condensed cream of celery soup and milk until smooth.
3. Arrange half of the sliced potatoes in the prepared casserole dish, then spoon over half of the soup mixture. Sprinkle with half of the shredded cheddar cheese. Repeat the layers with the remaining potatoes, soup mixture, and cheese.
4. Cover the dish with foil and bake for 45 minutes. Remove the foil and continue baking for 25 to 35 minutes more, until the potatoes are tender and the top is golden and bubbling.
5. Let the casserole rest for 10 minutes before serving so the sauce can settle and the layers hold together nicely.
Variations & Tips
For extra flavor: Add a small pinch of black pepper, onion powder, or garlic powder to the soup and milk mixture if you want a little more seasoning without changing the simple character of the dish.
For picky eaters: Swap the cheddar for a milder cheese like mozzarella or Monterey Jack for a softer flavor and gentler finish that children often enjoy.
Make-ahead tip: Assemble the casserole a few hours ahead, cover, and refrigerate. When ready to bake, let it sit at room temperature for about 20 minutes first so it cooks more evenly.
Potato tip: Slice the potatoes as evenly and thinly as possible so they soften at the same rate. A mandoline makes this quick, but a sharp knife works just fine too.
Cheesy top tip: If you like a deeper golden finish, place the baked casserole under the broiler for 1 to 2 minutes at the end, watching closely so the top does not burn.
