To make the beef: Prepare the fillet of beef by hammering it with a meat cleaver, the side of a meat cleaver, the side of a Chinese cleaver or a rolling pin. Slice it thinly, and put the pieces into a bowl. Season the beef with the light soy sauce, oyster sauce, salt, black pepper, and sugar. Set aside.
Heat a wok over high heat, and add 1 tablespoon groundnut oil. Add the garlic, and chile, and toss quickly. Then, add the spinach leaves, and toss so they warm for 5 seconds. Scatter them onto a large serving plate.
Heat the wok over high heat, and add the remaining groundnut oil. Add the beef slices, and stir-fry for 1 to 2 minutes, adding a dash of Shaohsing rice wine. Season with a pinch of dried chile flakes. To serve, lay the beef over a bed of the garlic-spinach leaves.
To make the mushrooms: Reheat the wok, add the oil, then add the oyster mushrooms and stir fry for 1 minute. Add a small splash of water to create some steam to help cook. Season with the black rice vinegar, and light soy sauce. Spoon the mixture over the beef, and serve.
*Can be found at specialty Asian markets.