Slice the steak into small strips. When slicing, it's very important to cut against the grain with flank steak to ensure it stays tender. Locate the parallel lines (muscle fibers) running along the steak, and cut in the opposite direction. This will ensure the steak isn't too chewy and stringy.
Add the soy sauce and toasted sesame oil (from the base section of the ingredients) to the steak and toss to combine, then toss with the cornstarch. Set aside.
Whisk together all ingredients for the sauce and set aside.
To a large skillet, add the butter and melt over medium heat.
Add in the broccoli florets and saute until the broccoli is a bright green color and fork tender. Careful not to overcook as it will continue to cook in the sauce later on.
Set the broccoli aside on a different plate.
To the same skillet, add in the beef and spread into a single layer.
Don't touch while cooking, but when the edges start to brown, you can toss and let cook on the other side.
When the beef is mostly cooked through, add in the sauce and mix to combine.
Once the sauce starts to thicken, add in the broccoli.
Toss everything to combine and simmer over low heat for a few minutes to let the flavors blend.
Garnish with green onion and sesame seeds. Serve over rice and enjoy!