In a small bowl, mix together the tamarind purée, the brown sugar, the fish sauce, and the oyster sauce to make your pad Thai sauce, and then set it to one side.
Add the rice noodles to a medium-sized bowl and cover with hot water. Allow the noodles to sit for 2 minutes, until they are al dente, and then drain them. Rinse them in cold water to stop the cooking process and then set them to one side.
Lightly beat the two eggs in a small bowl, and set them to one side.
Heat up a wok or a large frying pan to a high heat. Add the oil, and when it is shimmering add the sliced onion, the thinly sliced chicken breast, the chopped shiitake mushrooms, and the garlic cloves. Stir fry everything until the chicken is just cooked.
Add the eggs to the pan and scramble them, and then add 1 cup of bean sprouts, the finely sliced scallions, and the prepared rice noodles. Pour over the Pad Thai sauce and then toss everything together and allow it to heat through.
Serve hot and fresh topped with the remaining bean sprouts, the chopped peanuts, the lime wedges, and the fresh cilantro.