Cook the noodles according to package directions in rapidly simmering water until nearly al dente.
Plunge the asparagus and snow peas (or substitute) into the water. Cook for another minute or two, just until the veggies are bright green and the noodles done, then drain and transfer to a serving container.
In a small bowl, whisk together the sesame oil, hoisin sauce, vinegar, and soy sauce. Pour over the noodle mixture and toss together gently. Taste to see if you'd like to add any more vinegar or soy sauce.
Stir in the scallions and cashews and serve at once, passing around red pepper flakes or hot sauce if desired.