Prep the Beef: In a large bowl, season the thinly sliced flank steak with a pinch of salt and freshly cracked black pepper. Add the cornstarch and toss the beef until every piece is evenly coated. Set aside.
Mix the Glaze: In a small bowl, whisk together the dark brown sugar, low-sodium soy sauce, beef broth, minced garlic, minced ginger, sesame oil, and crushed red pepper flakes until the sugar begins to dissolve. Set aside.
Sear the Steak: Heat the vegetable oil in a large skillet or Dutch oven over medium-high heat. Once the oil is hot and shimmering, add the coated flank steak in a single layer (work in batches so you do not overcrowd the pan). Sear for 2-3 minutes per side until a golden-brown crust forms. Remove the beef from the pan and set it aside on a plate.
Simmer the Sauce: Lower the heat to medium. Pour the prepared glaze mixture directly into the skillet, scraping up any browned bits from the bottom of the pan. Let the sauce come to a gentle simmer for 3-4 minutes, allowing it to reduce and thicken into a sticky syrup.
Toss and Serve: Return the seared beef to the skillet. Toss the meat in the bubbling sauce for 1-2 minutes until it is completely coated, heated through, and glossy.
Plate: Remove from heat immediately. Serve hot over steamed rice alongside roasted vegetables, and enjoy!