In a small bowl, combine the tamarind paste and boiling water and leave to soak for 10 minutes, stirring frequently to break down the paste. Mix the resulting tamarind liquid with the soy sauce, sesame oil and rice wine or sherry.
While the tamarind is soaking, soak the rice noodles in boiling water for 4 minutes, or according to the packet instructions. Then drain, rinse under cold running water and set aside to drain thoroughly.
Heat the sunflower oil in a wok or very large frying pan and stir-fry the beef over a high heat for about 3 minutes or until cooked. Use a slotted spoon to remove the beef from the wok and set it aside.
Add the onion, lemongrass, chilli and garlic to the wok and stir-fry over a high heat for 1 minute. Add the snow peas, baby corn and mushrooms, and continue stir-frying for 2 minutes.
Return the beef to the wok. Add the tamarind liquid and the noodles and stir for about 1 minute to heat through. Serve immediately, while still hot.