Thai Prawns with Basil

K has been away for the last few days in New York, leaving me with the children. I can't usually be bothered to cook just for myself, hence the silence. She returned yetserday and described this as "very, very nice":

Uncooked prawns, peeled and de-veined
1 large handful of basil, roughly chopped (preferably Thai holy basil)
2 stalks lemon grass, finely sliced
2 cloves of garlic, finely sliced
6 red chillies (or to taste), chopped
1 small white onion, sliced vertically
2 Tbsp fish sauce
1 Tbsp sugar
3 Tbsp chicken stock or water
2 Tbsp cooking oil

Heat the oil in a wok or frying pan. Add the onion. Cook for a minute or two, then add the garlic, lemon grass and chilles. Cook for a further minute, then add the remaining ingredients. Cook until the prawns have cooked through and server over rice.