Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat. Add broccoli and cook, stirring often, for 3 minutes. Remove to a plate and place wok back over heat.
Add remaining 1 tablespoon of oil and beef to the skillet. Cook, stirring often, until browned, 3 to 4 minutes.
While beef is cooking, in a small bowl stir together the soy sauce, brown sugar, rice vinegar, sesame oil, garlic and cornstarch. Set aside.
Add broccoli back to the skillet along with the sauce. Stir together and cook until sauce is thickened, about 2 minutes.