Bring stock to the boil in a medium saucepan (lid on) over high heat.
 Add garlic, lemongrass, three-quarters of the chillies and half the kaffir limes leaves. Cover and boil for 3 minutes for flavours to develop. Reduce heat to
 medium-low. Add chicken and simmer, partially covered and turning chicken
 occasionally, for 5-6 minutes or until chicken is just cooked through. 
Transfer chicken to a plate to cool slightly. Set stock aside

Meanwhile, cook rice noodles according to packet directions.
 Drain and set aside. Shred chicken. Set chicken aside

Strain stock mixture into a large heatproof jug or bowl (discard the flavourings). Return strained stock to the saucepan. Cover and bring to the boil over medium-high heat. Add fish sauce, sugar, lime juice and remaining chillies to soup. Stir in bok choy and cook until just wilted

Divide noodles and shredded chicken among serving bowls. Ladle soup over 
the boiling chicken broth. Top with coriander and mint leaves and remaining shredded kaffir lime leaves. Serve with lime wedges and extra sliced red chilli