Heat oil in a frying pan or wok over medium-high heat, add onion and potato and cook, stirring 3 minutes until onion is soft. Add curry paste and cook, stirring 1 minute until fragrant. Add stock and coconut milk, bring to boil, reduce heat and simmer for about 5 minutes.

Add fish, lentils and coriander and simmer for 3 minutes, or until fish is just cooked.

Serve with lime wedges and garnish with extra coriander.