Soak the noodles in boiling water according to the packet instructions. Drain, refresh in cold water and set aside in a large bowl.
Blanch the asparagus in salted water, drain, refresh in cold water the cut into 5cm/2" lengths and add to the noodles.
Mix together the oil, lime juice, fish sauce, sugar and chilli and season to taste with salt.
Mix the sliced bell peppers into the noodles, stir in the dressing and scatter over the black onion seeds.