Thinly slice beef sirloin cross-grain. Make sure they are all about the same size. Slice broccoli head into florets. Mince fresh garlic.
In a bowl, combine all the ingredients under 'stir fry sauce' and mix well to combine. Make sure corn starch is completely dissolved.
Heat wok under high heat and add some vegetable oil to wok. Once oil is heated, add broccoli florets to wok and stir fry (move broccoli around) until it turns a darker green, or until almost done (does not have to cook completely as we will be cooking them again). Remove broccoli florets from wok and set aside.
Add more vegetable oil to wok. Once oil is heated, add minced garlic and beef sirloin to wok. Stir and move beef around every 10 seconds or so. Once beef is almost cook (takes only a few minutes), add broccoli florets back into the wok, then add sauce to the wok.
Stir everything around to combine. Once sauce is bubbly and all items are cook, remove promptly from heat. Avoid overcooking beef. The stir frying process goes quickly - step 3-5 should follow each other fairly quickly.
Serve with rice. Enjoy!