Place the noodles in a large bowl and completely cover with boiling water. Allow them to soak and soften while you prepare the sauce and vegetables, checking and tossing the noodles occasionally to separate them and see when they are ready (they will still have a bit of a bite), 6 to 7 minutes. Drain and rinse under cold water.
For the sauce: Combine the soy sauce, brown sugar, peanut butter, fish sauce, chile garlic sauce, lime zest and juice in a blender, along with the whites of the scallions, garlic and ginger. Blend to combine, 15 to 20 seconds. Set aside.
Heat the peanut oil in a large skillet over medium-high heat. Add the mushrooms and cook, stirring, until they begin to brown, about 2 minutes. Next, throw in the asparagus and red bell pepper, cooking for another minute or two. Push the veggies to the outer edge of the skillet and add the egg, stirring in the center of the skillet until the eggs scramble, then mix in with the vegetables. To ensure the vegetables keep their color and crunch, remove the vegetable-egg mixture from the skillet with a slotted spoon and set aside.
Add the noodles and the sauce to the hot skillet and stir to combine. Allow to come to a boil and cook, tossing to coat completely and allowing the sauce to thicken slightly, 1 to 2 minutes. Add the vegetables back to the skillet and carefully toss to combine. If the sauce is too thick, add 1/4 to 1/2 cup hot water. Remove the skillet from the heat and toss in the bean sprouts. Taste and adjust the seasoning. Transfer to a serving bowl and garnish with the chopped peanuts, cilantro, reserved scallion greens and as much sriracha as your heart desires.