Put the bean thread noodles in a medium bowl and the dried mushrooms (if using) in a small bowl, and cover each with warm water. Set aside to soften for 10 to 20 minutes. Meanwhile, beat the eggs in a medium bowl. Add the pork, fish sauce, salt, pepper, and green onion, and then set aside. When the noodles are soft and clear, drain and then rinse them well with cool water. Transfer to a cutting board and chop into 2-inch lengths. Drain the softened mushrooms, cut away and discard any hard lumps or stems, and cut into thin strips. Add the noodles and mushrooms to the eggs and pork and mix everything together as best you can, breaking up any large clumps of pork. Heat the oil in a medium skillet over high heat until a drop of egg sizzles and blooms at once. Add the egg mixture, lower the heat to medium, and cook for 3 to 4 minutes. As the omelet begins to set, use a spoon to work your way around the pan, pulling the cooked edges toward the center so that some of the uncooked egg spreads out in the hot pan.
Gently slide the half-cooked omelet onto a plate, place the hot pan upside down over the uncooked surface of the omelet, and invert the plate so that the cooked side faces up. Return the pan to the heat and continue cooking the omelet until the meat is done and the omelet is golden brown on both sides, 2 to 3 minutes. Transfer to a serving plate and serve hot, warm, or at room temperature.
source from: vietnameseportal