Optional garnish: Sesame seeds and thinly sliced green onions
White rice, for serving (optional)
In a bowl, briskly whisk together sesame oil, broth, cornstarch, soy sauce and brown sugar until sugar and cornstarch are fully dissolved. Set aside.
Heat oil in a wok or large, deep skillet over medium heat. Add onions and broccoli and cook for 3-4 minutes or until crisp-tender.
Add the minced garlic and ginger to the pan and saute for an additional minute, then transfer vegetables to a plate and cover to keep warm.
Wipe out the skillet with paper towels and turn the heat to high. Add another tablespoon of olive oil, then add the sliced beef to the skillet and season lightly with salt and pepper. Cook for 1-2 minutes per side or just until cooked through. You may need to do this in batches.
Add sauce to the skillet and simmer over medium-low heat for 2-3 minutes or until thickened. Add the broccoli and onions back to the skillet and stir to combine.
Serve over steamed rice and garnish with sesame seeds and thinly sliced green onions, if desired.
Wrap steak in plastic wrap or place in a ziploc bag and freeze for 20-30 minutes for easier slicing.
If the sauce is too thick, add 1-2 tablespoons of water or beef broth to thin it out.
Flank steak is preferred but you can also use top sirloin.