Place the prawns, cod and chillies in a food processor and blend until finely minced. Transfer to a bowl and stir in the fish sauce, tomato ketchup, salad onions and shredded basil.
Heat a little oil in a large nonstick frying pan. Using half the mixture, drop 4 large spoonfuls into the pan, smoothing them out into even-sized cakes. Cook for 3–4 minutes on each side until nicely browned and cooked through. Repeat to make a further 4.
Serve warm with lime wedges and greens or salad.