Place sirloin steak in a glass bowl or resealable plastic bag.
In a small bowl. whisk together soy sauce, rice vinegar, ginger, garlic, sugar, cornstarch and sambal oelek. Pour over sirloin steak and marinade for 20 minutes or up to overnight in the refrigerator.
Pre-heat wok or large non-stick skillet over high heat. Spray with cooking spray and place steak in pan - careful not to overcrowd.
After the beef starts to brown on one side, toss to the other side to brown. Should take about 2-3 minutes per side.
When beef is nicely browned, remove from pan and reserve for later.
Add chicken broth to the hot pan and use a wooden spoon to scrape up the browned bits at the bottom. Bring to a simmer then add broccoli. Cover and let broccoli steam for 3-4 minutes until bright green and crisp-tender.
Place the beef and any accumulated juices back in the pan along with the broccoli and stir in the oyster sauce. Toss until well combined and cook until warmed through, about 2-3 minutes.
Serve with brown rice and black roasted sesame seeds if you wish.
While cooking the beef, don't be afraid to walk away for a minute so that it can caramelize and brown. Home stoves are rarely as hot as restaurant cooktops which is why I don't suggest continually stir-frying/moving the beef around like you see with restaurant woks.