Blitz the lemongrass, garlic, ginger, shallots and chillies in a food processor until finely chopped, or chop by hand. Add the oil to a heavy-bottomed pan and fry for 3 minutes. Add the chicken thighs (and a little more oil if necessary) and fry for 2 minutes before adding the stock. Cook in the simmering stock for 15-20 minutes, adding the carrots after 5 minutes. Remove the thighs and carrots, leave to cool slightly and shred the meat. Meanwhile, pour some boiling water over the rice noodles in a bowl and let them sit until they have softened, about 4-5 minutes, then drain and set aside. Strain the broth through a sieve. Return to the pan and add the chicken, carrots and noodles, reheating if necessary. Add the soy sauce. Divide between two bowls and top with the coriander.