Put the chicken stock, coriander stalks, lime leaf, ginger, half the chillies and the lemongrass in a pan. Heat over medium heat to infuse. Add the mushrooms, chicken and beans, turn up the heat, and then bring to a simmer to warm the chicken. Add the rice vinegar, lime zest and juice and fish sauce, then taste and adjust the saltiness/sourness according to your taste. Serve in a bowl, garnished with the coriander leaves, remaining chillies, bean sprouts and fresh basil (if using), with lime wedges on the side.