This is the gratin recipe my mother makes often. It seems she learned this long ago from a friend. When I learned how simple it was, I think I may have become a little ungrateful .
Note: It's a bit more effort, but if you sift in the flour, it'll mix easily. But you don't have to.

If you'd like an even richer flavor, use 200 ml each of milk and heavy cream (in this case, omit the butter). Even with just milk, the dish will be creamy enough.
This is seasoned very lightly . Since you can add whatever ingredients you like, you may need to adjust the taste with consomme, which I didn't list. Recipe by Shiotomo