The grainy mustard and cheese whet your appetite It's great as a side dish or as a drinking appetizer, which goes well with wine. You can substitute the leeks with Shimonita leeks or Japanese leeks.
I didn't want to make gratin today, so I made an easy cheese bake instead. It was tasty! Since I liked it, I uploaded the recipe.
If you use the leeks right after washing and cutting, they will cook perfectly from the water on them while steam-frying. It's better to cook the leeks until they become soft.
Enjoy Japanese food around the world.