By chopping the oysters roughly, you can extract their tasty flavours into this dish. You won't mind the uneven sized oysters in it.
I used to add whole oysters, but my children didn't like it. I decided to chop them roughly and add to the gratin. They liked it and the taste itself became richer.
You can use different sized oysters as you will chop them later. The taste itself is very rich, so serve with a baguette or garlic toast.
Enjoy Japanese food around the world.