Begin by slicing the flank steak against the grain into thin strips, about ¼ inch thick.
In a small bowl, whisk together the packed brown sugar, low-sodium soy sauce, sesame oil, ground ginger, and cornstarch.
In a large wok or skillet, heat the olive oil over medium-high heat. Add the sliced flank steak in a single layer and sear for about 2-3 minutes.
Add the minced garlic and the chopped red and green bell peppers to the pan. Stir-fry for an additional 2-3 minutes.
Pour the prepared sauce over the meat and vegetables in the skillet. Stir to coat everything evenly and cook for another 2-3 minutes until the sauce thickens.
Remove the skillet from the heat and serve your Amazing Pepper Steak Stir Fry immediately over rice or noodles.