In a food processor put the onion, ginger, lemongrass, garlic, chillies, lime zest and juice and whizz to a rough paste. Add the fish sauce, shrimp paste, basil, coriander and oil and whizz again to quite a fine paste.
Heat a wok or large frying pan and add the paste. Fry for about 5 mins over a medium heat, stirring occasionally. Add the chicken and fry for a few more minutes, stirring to coat the chicken in the paste. Pour in the tin of coconut milk, bring to the boil and simmer for around 15-20 minutes until the chicken is cooked.
Check the seasoning – add salt/more lime juice if needed. Serve with rice.