Heat the butter in a saucepan, add the onion and sweat for about 5 minutes until soft but not coloured.
Mix together the chicken pieces and cornflour, and add to the pan with the garlic. Cook quickly, stirring occasionally, until golden brown.
Stir in the stock, chilli paste, coriander and mustard. Season with salt and pepper. Bring just to the boil, stirring, and simmer gently for about 10 minutes.
Just before serving, stir in the sesame seeds, and serve hot with pasta or potatoes.