Flatten the beef between cling film with a mallet, slice thinly, and marinate with soy sauce, oyster sauce, sugar, salt, and pepper.
Heat a wok with 15ml of groundnut oil and stir-fry the garlic, chilli, and spinach until the leaves are warmed through.
Wilt the spinach and transfer it to a serving plate.
Stir-fry the beef in the remaining oil for 2-3 minutes, then add rice wine and chilli flakes.
Place the cooked beef over the spinach.
Sauté the oyster mushrooms in oil for one minute with a splash of water, then season with vinegar and soy sauce.
Spoon the mushrooms over the beef and serve.