Place lemongrass, ginger, and garlic into the base of a large steamer.
Season chicken breasts with salt and pepper and place them on top of the herbs.
Steam the chicken over boiling water for 25 minutes until cooked through.
Boil water and sugar in a wok for 6 minutes until a syrup forms.
Add walnuts to the syrup for 30 seconds, then drain and set aside.
Boil chicken stock and cook pak choi and broccoli for 2 minutes.
Drain vegetables and plunge into ice water to stop the cooking.
Melt butter in a pan and fry the vegetables for one minute.
Add soy sauce and sesame seeds to the vegetables.
Slice the chicken and serve over the vegetables topped with walnuts.