For the dressing: To make the sweet and sour dressing: Combine the light soy sauce, lemon juice, orange juice, groundnut oil, sugar, and honey, in a bowl. Whisk to blend the ingredients, and then add the very finely chopped cucumber, and chile. Stir, and set aside. For the beef: Heat a wok over high heat, and add the groundnut oil. Add the beef to the hot wok, and stir quickly. Deglaze with the rice wine or sherry, and cook for a few seconds. Add the crushed dried chile flakes, light soy sauce, and white pepper. Cook to your preference: 1 minute longer for well-done or less than a minute for medium. Dress the serving plates with some of the baby spinach leaves, and sliced pak choy. Spoon some of the chile beef into the middle of the platter, and then drizzle generous amounts of the dressing over the top. Sprinkle the dish with finely diced mangos, and serve immediately.
You could use tail end of fillet for a less expensive cut.