First, prepare sumeshi (vinegared sushi rice) using any recipe that you already like.
While the rice is cooking, start preparing the king oyster mushroom 'fish.' Trim off the top and the bottom of the mushrooms then slice them lengthwise into 8.5 -10 mm thick slabs. Also, trim the sides if you like. Then, score diagonally on both sides but be sure not to go too deep or they will break apart.
Cook the mushrooms. Heat up a frying pan to a medium to medium-high heat (a non-stick surface is recommended). Pour in neutral oil and cook the mushrooms for about 2-3 minutes on each side until golden brown. Try not to move them too much for better coloring. Remove them from the pan and lay them flat–in a single layer–on a large plate or something similar. While still warm, season with soy sauce and toasted sesame oil. Set aside to marinate until the sumeshi is done.
Once the sumeshi is ready (it should be cooled down to around body temperature), start assembling. Dampen your hands and grab a small amount of rice in one hand. Toss and roll gently to form an oval shape (don’t squeeze!). Place the marinated mushroom “fish” on top. Shape it again with gentle pressure. Repeat the process.
Right before serving, prepare your shiso leaves. Clean the leaves under running water and gently pat dry with a towel. Cut off the stem and roll all the leaves together (like making a sushi roll) before slicing them thinly. To serve, top your sushi with a pinch of thinly sliced shiso leaves and a tiny amount of neri ume (about 1/2 tsp for each nigiri.)
