Chop the king oyster mushrooms into thin strips, and bacon in 1 cm wide strips. Slice the leek thinly. Boil the pasta according to instructions on the package.
Heat butter in a pot (or a frying pan). Add bacon. After the bacon gives out some oil, add the leek and stir-fry over medium heat. Add the mushrooms and continue to fry until soft.
When the mushrooms are soft, turn the heat off. Add flour and stir until the flouriness is gone (about 30 seconds).
Add milk and mix thoroughly. Turn the heat back on and simmer over medium heat until thick. Add the [A] ingredients to finish the white sauce.
Combine 1 ladle of white sauce, prepared in Step 4, with the cooked pasta.
Transfer the ingredients from Step 5 into a gratin dish. Pour over the rest of the sauce and top with heaps of pizza cheese.
Bake in 250℃ preheated oven until browned on top (about 15 minutes).
Story Behind this Recipe
For a while I wanted to create a gratin that makes the most of the texture of king oyster mushrooms, but after many tryouts I always ended up with a dish that has a very strong smell of mushrooms because I used a lot of them. So, I added some Japanese leek and cheese to dampen their distinctive aroma. The result was this very thick and rich gratin.