Slice the beef really thin (popping it in the freezer for 15 minutes will help the steak firm up a bit so it makes it easier to slice) add it to a bowl, add one Tbsp of soy (from the total amount above) along with one teaspoon of cornstarch, mix well and set aside.
In a large skillet, add a touch of oil, preheat over high heat, then in batches, sear the beef in batches for just a minute or two before removing to a plate and set aside.
Add another drizzle of oil then add the peppers and onions, saut� for 2 minutes while making the sauce.
In a bowl or measuring cup, add the soy, water, brown sugar, mirin, vinegar, garlic, ginger and cornstarch, whisk well and set aside.
After 2 minutes, add the seared beef back in along with the sauce, cook all together about 2 minutes, add a small splash of water if the sauce is a tad thick, then serve with scallions and starch of choice.
