Heat the oil in a wok or saucepan and stir-fry the green curry paste over a medium heat for 3–4 minutes or until fragrant.
Add the coconut milk, stock or water, fish sauce and sugar or honey. Simmer over a medium heat for 3–4 minutes or until the sugar has dissolved. Add the fish and the water chestnuts and cook for another 4–5 minutes. Stir gently a few times during cooking. Taste and adjust the seasoning.
Add the tomatoes and kaffir lime leaves in the last few seconds, taking care not to let the tomatoes lose their shape.
Spoon into 4 serving bowls and garnish with basil leaves and chilli slices.