Potato gratin

Milk curdles when heated in the presence of salt and/or acid. Potatoes are mildly acidic, and presumably there was salt in your gratin. The more fat in the milk, the less likely it is to curdle, so one way to reduce or prevent the separation is to use higher-fat dairy next time.

Another way is to introduce starch into the liquid, which will inhibit curdling. You could use a thin bechamel sauce instead of milk (there's starch from the flour in the roux) or you could parcook the potatoes halfway in barely simmering milk before you build the gratin (some starch from the potatoes will mix with the milk).