Prepare sushi rice in a rice cooker and follow manufacturer instructions carefully. When cycle has completed, transfer rice to a non-reactive bowl and fold in olive oil, walnut oil, sherry and rice wine vinegar while rice cools. Cover bowl with a damp dish towel and reserve at room temperature.
Heat a cast iron skillet over medium-high heat. Pat steak dry and liberally season. Film pan with one tablespoon grape seed oil, heat until oil begins to shimmer and then carefully lay the steak in the pan. Sauté for approximately 6 minutes per side for medium rare and check the temperature with an instant read thermometer (125 degrees). Remove the steak and allow it to rest warm for a minimum of 5 minutes. Once rested, slice across the grain into 1/8 inch pieces.
Add butter, remaining grape seed oil and mushrooms to the cast iron pan and sauté over medium-high heat, until edges begin to color (about 3-5 minutes). Remove from heat and let cool.
To plate, form 12 nigiris by pressing approximately one tablespoon of rice into the palm of your hand, or divide rice into 6 small bowls. Layer each serving with roasted mushrooms and a slice of steak. Garnish with shiso, Maldon salt and freshly ground pepper.