I made a gratin recently with thinly sliced potato, celeriac, leek, feta cheese and sage. There was no cream or milk in the recipe, just water added to the veggies and it was delicious.
cut up sweet potatos celeriac,leeks and carrots, smother with olive oil add herbs of choice ( I use rosemary and thyme) and roast in the oven, yummyy!!