Heat the oil in a wok or large frying pan and add the chicken. Cook over a medium-high heat for 4-5mins, stirring occasionally, until the chicken is sealed and browned. Add the onion and garlic and cook for a further 2mins.
Add the red pepper, lemon grass, ginger, coconut milk, chicken stock and red curry paste. Stir in the fish sauce or soy sauce and coriander.
Bring to the boil, then reduce the heat and simmer for 20-25mins, until the chicken is cooked, cutting into a thick chunk to check it is cooked through. Remove the lemongrass from the sauce.
Serve, garnished with more fresh coriander and accompanied by boiled or steamed rice or noodles. If you like things a bit spicier, you could add some sliced red chilli when serving.
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