How to Make It
In a large pot, heat the cooking oil over moderately low heat. Add the onion and cook, stirring occasionally, until it is translucent, about 5 minutes. Add the garlic, coriander, ginger, black pepper, and cayenne; cook, stirring, for 30 seconds.
Add the broth, coconut milk, fish sauce, salt, and lime zest. Bring to a simmer, stirring occasionally. Reduce the heat and simmer, partially covered, for 10 minutes.
Meanwhile, in a large pot of boiling, salted water, cook the fettuccine until just done, about 12 minutes. Drain.
Add the chicken to the soup and simmer until just done, about 1 1/2 minutes. Remove the pot from the heat and stir in the fettuccine, lime juice, and cilantro, if using. Serve the soup in bowls with a fork and spoon.
Asian fish sauce is available at Asian markets and some supermarkets.