First, slice the sirloin into small chunks, about 1 inch long, like in the photo. Set aside.
Next, mix the marinade together in a large mixing bowl. Add the soy sauce, sesame oil, hoisin, rice wine vinegar, brown sugar, ginger, and sesame seeds in a large bowl. Mix until combined.
Add your steak to the bowl with the marinade in it. Mix around to ensure all the steak is covered by the sauce. Let sit and marinade in the fridge for 30 or so minutes. You can be flexible with the timing depending on what works for you, but the longer the steak sits in the marinade, the more flavorful your finished dish will be.
While the steak is marinated, cut your broccoli into small and almost bit size florets and slice the scallions on the bias into about 1 inch long segments. Preheat oven to 425.
Line two baking sheets with tinfoil to make for a super easy cleanup. After the steak has marinated, use a slotted spoon to transfer the steak to the pan and spread it out evenly between the baking sheets, which will come out to about one pound of steak per sheet. Leave the extra marinade in the bowl.
Add the broccoli into the large bowl with the marinade, and mix around until slightly coated. After this step, there should still be extra marinade still in the bowl that we will cook down into a sauce.
Fill in the gaps on the baking sheets with the broccoli and sliced scallions. Sprinkle extra sesame seeds onto the top of the food. This step is optional, but I'm a huge fan of sesame and love the extra texture and flavor they bring to the dish.
Cook the food for 7-8 minutes in the oven. You can totally add both sheets to the pan at the same time, but if you do, just rotate them around after 4 minutes so each has a chance to be on the middle, not under the other pan to get a nice sear or cook on the steak.
While your beef and broccoli are cooking in the oven, add the extra marinade to a small saucepan on medium heat.
Mix together your cornstarch and water. Once your sauce begins to simmer in the pot, add the cornstarch mixture and stir until thickened. The sauce should be a little bit thinner than honey.
Serve and enjoy! I usually love to eat this recipe with rice and some chopped scallions on the top, but you could enjoy over spinach, rice noodles, or even on its own!