Place dry rice noodles in a large bowl.
Cover with hot tap water and let noodles soak for 30 minutes to 2 hours.
In a small bowl, stir together soy sauce, lime juice, brown sugar and cayenne pepper. Place beaten eggs, onion, garlic, prawns, bean sprouts, green onions, and cilantro in separate bowls to be gradually added to the stir-fry.
Heat half of the vegetable oil in a wok or very large skillet over medium-high heat.
Add eggs and cook, stirring, until the eggs are firmly scrambled.
Remove scrambled eggs from heat and place in a bowl.
Drain the noodles into a colander.
Add the remaining oil to the wok or skillet and heat over medium-high heat.
Add minced onion and garlic and cook, stirring constantly, until just golden (approximately 1 minute).
Add prawns and stir-fry for 1–2 minutes or until thoroughly cooked.
Add soy mixture, 2 cups of bean sprouts, green onions, noodles and water.
Cook, stirring constantly, until the noodles are tender and most of the liquid has been absorbed.
Return the scrambled egg to the pan along with the cilantro and toss to mix.
Remove noodle mixture from heat and empty into a large bowl or onto a platter.
Top noodles with remaining bean sprouts.