Cook pasta according to directions, drain and rinse.
While pasta is cooking, combine lime juice, fish sauce, oil, and sugar in a large bowl. Add the sliced onions and the pepper to the dressing to soak while you prepare the rest of the vegetables.
Combine the noodles, chicken, tomatoes, cucumber, and cilantro with the dressing and toss to combine.
Top with the peanuts for serving.
Notes
I like to add the onions and the pepper to the dressing to quick pickle the onions and to evenly distribute the heat from the pepper.