Heat the oven to 425°F convection roast or 450°F conventional with the rack in the upper middle.
While the oven is heating up, chill the flank steak in the freezer for about 15 minutes to make it easier to slice.
Cut the flank steak in half lengthwise, along the grain of the meat. Next, cut the beef crosswise, against the grain, into thin slices that are ¼-inch thick.
In a large bowl, season the sliced flank steak with 1 tablespoon of arrowroot powder, ½ teaspoon of Umami Stir Fry Powder or Diamond Crystal kosher salt, and 1 tablespoon of avocado oil. Toss well to combine.
Grab a large rimmed baking sheet and lightly coat it with avocado oil or avocado oil spray. Arrange the seasoned meat in a single layer on about two-thirds of the pan.
Toss the broccoli with ½ teaspoon of Umami Stir-Fry Powder or Diamond Crystal kosher salt and the remaining tablespoon of avocado oil.
Arrange the seasoned broccoli in a single layer next to the beef and pop the tray in the oven for 10 minutes.
While the sheet pan is cooking, whisk together All-Purpose Stir-Fry Sauce and 1 teaspoon of arrowroot powder.
When the cooking time is up, the beef and broccoli should be mostly cooked through. Take the pan out of the oven and pour the sauce on top of the sheet pan, tossing everything well. Rearrange the beef and broccoli to be in a single layer again.
Turn on the broiler of the oven and broil the sheet pan for 3 to 5 minutes or until the sauce thickens and the meat and broccoli are browned in parts.
Sprinkle on the toasted sesame seeds and sliced scallions, if desired, and serve immediately.