Begin by slicing the sirloin steak thinly against the grain. Freezing the steak for 30 minutes beforehand makes slicing easier and results in more tender pieces. Peel the fresh ginger using a spoon to minimize waste, then slice into thin matchsticks. Mince the remaining ginger and garlic. Chop the onion, broccoli, spring onions, and prepare the mangetout and spinach.
Heat the neutral oil in a wok or large frying pan until shimmering hot. Add the ginger matchsticks and fry for 2-3 minutes until golden and crispy. Remove with a slotted spoon and drain on kitchen paper.
Season the thinly sliced beef with salt and pepper. In the same wok, add half of the beef and stir-fry for 3-5 minutes until browned. Remove and set aside. Repeat with the remaining beef.
If necessary, add a bit more oil to the wok. Add the chopped onion and cook for 2-3 minutes until softened. Add the minced ginger and garlic, frying for an additional 30 seconds. Add the broccoli, spring onions, and mangetout, stir-frying for about a minute.
In a small bowl, mix together the soy sauce, oyster sauce, rice wine, and brown sugar. Pour this sauce into the wok with the vegetables and bring to a boil. Let it bubble and thicken for about a minute.
Return the cooked beef to the wok, stirring to combine. Add the fresh spinach and cook for another minute until wilted. Serve the stir fry over boiled rice or noodles, topping with the crispy ginger matchsticks and a sprinkle of chili flakes.