Combine steak strips and cornstarch in a large zip-top bag. Seal and shake to coat steak. Set aside.
To a large skillet over medium-high heat, add olive oil. Once hot, add steak and cook until cooked through, 5-7 minutes, turning steak occasionally. Add broccoli.
Meanwhile, stir together the garlic, sesame oil, soy sauce, water, and brown sugar.
Pour over beef and broccoli, reduce heat to medium, and let simmer until broccoli is tender but still crisp.
If a thicker sauce is desired, stir 2 tablespoons cornstarch into an equal amount of cold water and stir into the sauce. Let simmer until thick.
Sprinkle with sesame seeds, if desired, and serve. Enjoy!