Whisktogether the olive oil, miso paste, soy sauce, Dijon mustard, and minced garlic. Pour mixture in ziploc bag with steak and let sit at room temperature for 20 minutes.
Make garlic soy sauce by whisking those ingredients in a small bowl and set aside.
When ready to cook, remove steak from marinade and cook in a hot skillet over medium-high heat for about 3 minutes per side. Remove steak from pan, wrap in aluminum foil, and let rest.
Meanwhile, add 1 Tablespoon sesame oil and the bok choy to the same skillet. Cook for 2 minutes, flip them over, add 1 Tablespoon water, cover pan, and cook for another 2 minutes.
Uncover pan and cook another 2 minutes until most of the liquid has evaporated and the bok choy is soft to the touch. Remove from pan to your serving platter.
In the same skillet, add 1 Tablespoon butter and the mushrooms and sauté for 2 minutes. Remove from pan to your serving platter.
Take steak out of the aluminum foil and spread out on serving platter with the bok choy and Enoki mushrooms. Pour garlic soy sauce over everything and serve immediately.