• 900 g (2 lb) small whole fish such as sea bass or cod, cleaned, or 450 g (1 lb) boneless fish fillets
• 4 tablespoons fish sauce or light soy sauce
• 450 g (1 lb) fresh watercress
• 1½ tablespoons plus 400 ml (14 fl oz) groundnut (peanut) oil
• 2 tablespoons coarsely chopped garlic
• 2 tablespoons finely sliced shallots
• 1 or 2 small red or green fresh Thai chillies, seeded and chopped
• ½ teaspoon freshly ground black pepper
• 2 teaspoons sugar
• plain flour, for dusting

If you are using whole fish, make 3 deep cuts on each side, then rub with 3 tablespoons of the fish sauce and let it marinate for 1 hour. If you are using fish fillets, cut them into four equal portions, coat with the fish sauce and marinate for 1 hour.

Wash the watercress thoroughly and remove any tough stems. Then spin dry in a salad spinner or drain well in a colander. Finish by drying in a clean linen towel.

Heat a wok or large frying pan over high heat until it is hot. Add the 1½ tablespoons of oil, and when it is very hot and slightly smoking, quickly add the garlic, shallots, chillies and pepper and stir-fry for 10 seconds. Then add the watercress and continue to stir-fry for 2 minutes over high heat, until slightly wilted. Now add 1 tablespoon of the fish sauce and the sugar, and stir well. Arrange on a warm platter.

Clean the wok and reheat it over high heat. Add the remaining oil. While the oil is heating, remove the fish from its marinade and dry with the kitchen paper. Dust with flour, shaking off excess. When the oil is very hot, slowly add the fish or fillets and deep-fry them for 5 minutes until they are golden brown. Remove the fish or fillets with a slotted spoon, drain immediately on kitchen paper and serve on top of the stir-fried watercress.