noun
potatoes au gratin
A baked dish of sliced potatoes in a creamy sauce, often with cheese, similar to Western potato gratin.
今夜の夕食はポテトグラタンにしよう。
Let's have potatoes au gratin for dinner tonight.
Variant with interpunct, sometimes used for clarity.
グラタン is a broader term for any gratin dish; ポテトグラタン specifically features potatoes as the main ingredient.
Borrowed from English 'potato gratin' or French 'gratin de pommes de terre', adapted into Japanese as ポテトグラタン.