Heat 1 tablespoon vegetable oil in a large skillet or wok over medium-high heat until shimmering. Add half the beef in a single layer, if possible, and cook until browned on one side and crisp around the edges. Turn the beef slices and continue cooking for about 2 minutes more, or until no longer pink. Remove to a plate and keep warm. Repeat with another tablespoon of oil and more sliced beef until all meat is browned.
Add 1 tablespoon of oil into the now-empty pan. Add the onion, carrots and broccoli and stir fry for 4-5 minutes. Reduce the heat to medium and add the garlic, ginger and crushed red pepper flakes. Sauté, stirring constantly until the garlic is fragrant, about 1 minute.
Return the cooked beef to the skillet, along with any accumulated juices. Whisk the sauce mixture again to blend then add to the skillet. Cook, stirring frequently, until the sauce thickens, about 3 to 5 minutes.
Remove from the heat and serve over hot cooked rice, garnished with sliced scallions and toasted sesame seeds.