Cut the apple and onion into 1 cm thick slices. Remove the apple's core. Mix the wasabi and soy sauce together.
Lightly oil the frying pan with oil, then cook the apples and onions to your desired softness. Once cooked on one side, flip over and cook the other.
If you prefer them to be soft, cook until crispy on one side, flip, sprinkle the sake on top, cover with a lid, and steam.
Once baked, turn off the heat, top with butter, and sprinkle the wasabi soy sauce on top. Turn the heat back on. Once it begins to sizzle, remove from heat.
A closeup shot.
If you cook them in a small paella skillet, you can serve them directly on the skillet. It's spicy, yet sweet, so it's great as a side dish or as a drinking snack.