Heat the oil in a deep frying pan or wok, stir in the curry paste and fry for 1 minute. Add the chicken, sweet potatoes and lentils and stir to coat in the paste, then pour in the stock and coconut milk. Bring to the boil, then simmer for 15 minutes.
Tip in the peas, bring back to the boil and simmer for a further 4-5 minutes. Season to taste before serving.