In a medium bowl, combine the soy sauce, mirin, and cornstarch. Pat the pieces of beef dry and add them to the bowl. Gently stir to completely coat the beef in the marinade. Set aside for 10-15 minutes.

In a glass measure or small bowl, add all the ingredients for the sauce. Whisk well to combine. Set aside.

Heat 1 ? tablespoons of oil in a wok or large heavy-bottom skillet over HIGH heat. When the oil is shimmering, add the steak to the pan in an even, single layer. Cook undisturbed for 30 to 45 seconds or until the bottom side is lightly browned. Flip and continue to cook, stir-frying for an additional 30 seconds or until almost cooked to your liking. Immediately remove the beef from the pan and transfer to a plate. Set aside.

Return the pan to the stove over MEDIUM heat. Add the remaining 1 ? tablespoons of oil to the pan. When the oil is hot, add the onions and peppers. Season with a pinch of salt and pepper. Stir fry the vegetables for 3-4 minutes or until they are starting to soften. Add the garlic and ginger to the pan. Stir-fry for 30 seconds.

Whisk the stir-fry sauce to ensure everything is combined and the cornstarch is fully dissolved. Add the stir-fry sauce to the pan and immediately stir the mixture. Reduce the heat to maintain a simmer and cook, stirring occasionally, for 1 minute or until the sauce resembles a thick syrup and coats the back of a spoon.

Add the cooked steak back to the pan along with any juices pooled on the plate. Quickly stir-fry to coat the steak in the sauce, about 30 seconds. Immediately remove from heat.

Transfer to a large serving platter or individual plates. Top with scallions and sesame seeds if using and serve with rice or noodles.