Gratin is very popular dish in Japan which is adapted from French cuisine. There are two types of gratin in Japan. First one is macaroni gratin and the second one is potato gratin. Both use white sauce; Bechamel sauce.
Macaroni or potatoes are placed into oval casserole together with Bechamel sauce and other ingredients; vegetable, meat and seafood and topped with grated cheese. Then, the casserole is put into an oven to bake.
Gratin is one of the few dishes in Japan which Japanese use cheese for cooking at home. There is a similar dish called "Doria" which contains rice instead of macaroni/potatoes.
If you are in Japan and miss cheese, gratin is a good choice to order at restaurants. Or if you have microwave oven at your accommodation facility, you can buy the frozen one at any supermarket in Japan.