Whisk rice vinegar and sugar together and pour over the finely sliced onion rings. Allow to marinate for 1 hour, tossing frequently.
Cook noodles according to packet directions. (Usually, rice noodles need only to soak in boiling water for 5 minutes, otherwise, boil for 1-2 minutes then drain immediately and rinse under cold water.)
Cut off the tough stalks of the asparagus, and cut remaining stalks into 2cm lengths. Simmer asparagus in salted water for 2 minutes until bright green and crispy tender. Rinse in cold water to refresh.
Toss noodles with reserved onion and vinegar mixture while still warm. Then, using kitchen scissors, cut noodles into manageable lengths (10cm).
Add the cooked asparagus to noodles along with chopped mint, coriander, cucumber, spring onions, tomatoes and roasted peanuts and toss thoroughly.
Whisk lime juice, fish sauce, oil and chili flakes together and drizzle over the noodle salad. Serve at room temperature.
