Rinse the oysters in salted water and drain. (If you are using frozen oysters you can use them as-is.) Cut the bacon up.
Boil the spinach briefly, drain and refresh in cold water. Squeeze out excess water and cut into pieces. Slice the onion thinly, and loosen the shimeji mushrooms apart.
Spread 1 tablespoon of olive oil in a frying pan, and sauté the bacon, then the onions. Add the shimeji mushrooms and oysters, and sauté them too.
Push all the ingredients to one side of the frying pan. Melt the butter in the open space in the pan over low heat. Add the flour and mix it with the butter. Cook the butter-flour mixture well.
Add the spinach and mix the ingredients that were pushed to the side back as you sauté everything.
Add the milk and soup stock to mix well. Simmer until the sauce has thickened. Adjust the seasoning with salt and pepper.
Transfer the contents of the frying pan to a heatproof dish, top with as much cheese as you like, and bake in an oven until the top is browned.
You can make this into a rice gratin! Put a small amount of rice in a heatproof dish, and pour the Step 6 mixture over it. Bake as in Step 7. It tastes rich and delicious too!
Story Behind this Recipe
I learned how to simpify the bechamel sauce steps for a gratin in this way a long time ago in a cooking class. Apparently there are many similar methods, but this one is the easiest and is foolproof I think.
None in particular. You can adjust the amounts of the filling ingredients and milk to taste. The more oysters you add the tastier! Try using 1/2 a broccoli instead of the spinach. Or try using king oyster mushrooms instead of the shimeji mushrooms, and so on. You can add anything you like! You can make macaroni gratin in the same way!