Rinse rice several times in running water until water gets clear. Drain in a colander. Then put the rice and 1.5 cups water in a sauce pan with a lid or rice cooker. Cook rice according to package directions.
In a bowl, mix together miso, wasabi and mirin.
In another bowl, mix together rice vinegar, sugar and salt.
Season beef with black pepper. In a skillet, heat olive oil over medium-high. Cook both sides of beef until brown. Take the beef out of the skillet and place on a cutting board. Let rest for a couple of minutes. Then cut each beef steak into half crosswise.
Slice mushroom into four. In the same skillet, put mushrooms and cook both sides of mushrooms until lightly brown and tender.
When the rice has cooked, transfer the rice into a medium bowl. Add rice vinegar mixture to the bowl and mix to combine (don’t mix too much).
Make beef sushi, Place one beef steak on one sheet of plastic wrap, spread the miso mixture on the beef and put about 1 tablespoon of sushi rice. Wrap it up tightly. Repeat this with the rest of beef, miso mixture (save some for mushroom sushi) and rice, and make 3 more beef sushi.
Make mushroom sushi. Place one slice of mushroom on one plastic wrap, spread the miso mixture and put about 1 tablespoon of sushi rice and wrap it up shaping into a bowl. Repeat this with the rest of mushrooms, rice and miso mixture, and make 3 more mushroom sushi.
To serve, remove the plastic wrap from both beef sushi and mushroom sushi and discard the plastic wrap. Put some scallions and sesame seeds over the top of beef sushi and place on a serving plate. Put some sesame seeds over the top of each mushroom sushi then wrap each sushi with 1 strip of nori then put scallions on top. Place on the serving plate.