Cut the stem off the shiitake mushroom. Discard the hard inedible end of the stem, and thinly slice the rest. Shred the crab stick. Slice the onion thinly, then cut in half.
Mix the mushroom stem, crab stick and onion with mayonnaise (add pepper to taste).
Stuff the mixture into the inverted mushroom cap. Put the cap in a foil cup.
Bake in an oven for 5 to 7 minutes. When the mayonnaise is bubbly, it's done. (Adjust the cooking time depending on the size and thickness of the mushroom cap.)
You can also bake it topped with cheese. This is very convenient for bentos. You can just put it in foil cup. (It's in the front middle of the bento.)
Story Behind this Recipe
This is a recipe my mother used to make.
You can easily make just one, so it's great for bentos. While you are baking it in the oven, you can toast bread for breakfast at the same time. Since a delicious liquid will come out of the mushroom while it's baking, it's best to put it in an aluminium foil cup or on a piece of foil.