While the steak is chilling, mix together the sauce.
In a mixing bowl, whisk together the beef stock, low sodium soy sauce or coconut aminos, brown sugar, cornstarch, toasted sesame oil, hoisin sauce, rice vinegar and sriracha (if using) until well-combined. Set aside.
1 cup beef stock, 5 tablespoons low-sodium soy sauce, 3 tablespoons brown sugar, 2 tablespoons cornstarch, 1 tablespoon toasted sesame oil, 1 tablespoon hoisin sauce, 1 tablespoon rice vinegar, 1 to 2 teaspoons sriracha
Remove the steak from the freezer or fridge and slice into ¼ inch strips making sure to cut against the grain. Place steak strips into a ziploc bag or bowl and toss with ¼ cup of the stir fry sauce. Set aside.
1 pound flank steak
In a large skillet or wok, heat 1 tablespoon of the olive oil over medium-high heat. When the oil is hot, add in the broccoli florets and sauté for 2 to 3minutes. Add ⅓ cup water and place lid on skillet. Steam broccoli until crisp tender or as tender as you want. Remove the broccoli to a plate and set aside. Wipe the skillet clean.
3 tablespoons light olive oil, 4 to 5 cups broccoli florets, ⅓ cup water
Heat 1 tablespoon of olive oil in the skillet or wok. When the oil is hot, add in the sliced steak and cook until no longer pink, about 1 to 3 minutes per side. Transfer the steak to a plate and set aside. *Cook the steak in 2 batches for more even cooking.
Heat the remaining 1 tablespoon of olive oil in the skillet and add the minced garlic and ginger. Cook for 30 seconds, stirring continuously to keep the garlic from burning.
3 tablespoons light olive oil, 1 tablespoon minced garlic, ½ - 1 tablespoon fresh ginger
Add the stir fry sauce to the skillet and bring to a boil. Reduce the heat to medium-low and cook, stirring often, until the sauce thickens, about 2 to 4 minutes.
Add the steak to the skillet and toss to combine with the sauce. Stir in the broccoli and heat another 1 to 2 minutes to warm the broccoli. Sprinkle with sesame seeds, green onions, red pepper flakes (all optional) and serve over rice.
3 teaspoons sesame seeds, 3 green onions, red pepper flakes, cooked rice for serving