[To make the white sauce] Heat a frying pan over medium heat, melt the butter in the pan, and mix with a bit of flour. Once the butter has combined with the flour, add the milk a little at a time while mixing so that it doesn't form lumps. Season with salt and pepper.

[To make the meat sauce] Heat some oil in a frying pan and add the finely chopped onion. Stir-fry until translucent. Add the ground meat and cook until it changes color. Add the meat sauce and stir together.

Thinly slice the potato with a slicer and boil until soft. Boil the gyoza skins one at a time so that they do not stick together.

In a gratin dish, layer the meat sauce --> potatoes --> white sauce --> Parmesan cheese --> gyoza skins. Repeat this process twice. Finally, spread the meat sauce over the top and sprinkle with pizza cheese.

Bake in an oven preheated to 230℃ for about 15 minutes.