Combine the soy sauce, garlic, brown sugar, sesame oil, rice wine vinegar, ginger, sriracha, salt and pepper in a small bowl. Whisk to combine.
Put your cubed flank steak into a large Ziploc bag and pour your marinade over it.
Shake the bag around to evenly distribute the marinade over the meat. Let it marinate for 1 hour.
Preheat your oven to 425F and line a baking sheet with tin foil.
Once your meat is done marinating, use tongs to transfer it to the prepared baking sheet, leaving the majority of the marinade behind.
Toss your broccoli florets into the marinade and then transfer the broccoli to the baking sheet with the steak. Again, leaving the majority of the marinade behind so you can boil the rest to make a sauce.
Bake in the oven for about 10 minutes, or longer to reach your desired internal meat temperature. 10 minutes is the sweet spot for me for flank steak, it cooks the meat to about medium/medium-rare. You want an internal temperature between 135-145F.
While the meat cooks, transfer the remaining marinade to a small saucepan over medium-high heat. You need to bring this to a boil, since it was in contact with raw meat.
Once the marinade is boiling, whisk in your corn starch. Continue to whisk until the sauce is thickened. The sauce will be pretty thick.
Once the meat is cooked, remove from the oven and use some pan drippings to thin the sauce out. You want that meaty flavor!
Drizzle your sauce over your beef and broccoli and top with sesame seeds and green onions.
Serve alone or over rice or noodles.