In a small mixing bowl add 1 tablespoon cornstarch and ½ cup water and stir until the cornstarch is completely dissolved.
Add ¼ cup low sodium soy sauce, 2 tbsp brown sugar, 1 teaspoon grated fresh ginger, 2 cloves minced garlic, and ¼ teaspoon red pepper flakes, then stir and set aside.
Cut the broccoli head into bite-size pieces and set aside.
Slice the flank steak thinly (about ¼-inch thick) against the grain and set aside.
Preheat the skillet over medium heat and add a tablespoon olive oil. Add the broccoli florets and stir. Add about 2 tablespoons water, cover with the lid, and let it cook for a few minutes to soften up. Then remove the lid and finish cooking until all the water is absorbed. Transfer the broccoli to a plate and set aside.
Add another tablespoon of olive oil to the pan and increase the heat to high. Add the beef to arrange it in a single layer. Cook it for a few minutes, stirring occasionally, until cooked through. If the meat starts to release too much liquid, just scoop out the liquid and discard.
When the beef is cooked, reduce the heat to medium/low and add the sauce mixture. Mix it with the meat and let cook for a couple of minutes, until it starts to bubble and get thicker. Add broccoli and stir again to combine. Sprinkle some sesame seed on top and serve with a bowl of rice.