Place steak and mushrooms in a large bowl and sprinkle with cornstarch, soy sauce and a generous amount of salt and pepper. Toss to coat and set aside to marinate at room temperature for 15 minutes, or in the fridge for up to 5 hours.
Toast sesame seeds in a dry skillet until golden brown, shaking pan frequently to prevent burning. Remove from heat right when they become fragrant but haven't gotten to brown yet. Let cool in pan and finish toasting continuing to stir every once in a while.
Combine orange zest, orange juice, vinegar, sherry, sriracha, and oyster sauce in a medium bowl and mix well.
Heat half of the peanut and sesame oils in a large non-stick frying pan over high heat. Once shimmering add half of the meat-mushroom mixture. Spread in an even layer and let cook, undisturbed until lightly browned. Give a stir and let brown on other side. Remove from pan and repeat with remaining oils and beef mixture.
Return first batch of beef to the pan and add ginger and garlic and stir-fry for about 30 seconds or until ginger is beginning to brown. Add sauce to pan and cook, stirring, until sauce starts to boil.
Sprinkle sesame seeds and green onions over top and serve with steamed rice and sauteed bok choy or spinach.