In a small bowl, whisk together all of the ingredients for the marinade and then place the marinade, steak and onions together in a zip-seal plastic bag and let marinade in refrigerator for 30 minutes, shaking the bag every 10 minutes (so 3 times total.)
Ten minutes after the steak is marinating, peel and spiralize the turnip, using Blade C. Place into a food processor and pulse until rice-like. Then, slice the shiitake mushrooms and set aside. Place a large skillet over medium heat and add in half of the olive oil and then, once heated, add in the garlic, ginger and red pepper flakes. Cook for 30 seconds and then add in the mushrooms, cooking for 2-3 minutes or until they begin to wilt. Add in the turnip rice, stir and cover for 5 minutes, uncovering occasionally to stir, or until rice texture is to your preference. Divide into four bowls and set aside, covered.
Place the large skillet back over medium heat. Remove the beef from the bag of marinade and set marinade aside. Add the beef in single layers to the pan, along with 1 tablespoon of the leftover marinade and cook for 3 minutes and then flip, cooking another 3 minutes or until beef is cooked. I like it browned, so I cooked a few minutes longer until dark brown.
Divide the beef atop the bowls of mushroom turnip rice, pouring any pan juices on top.