Bring a saucepan of water to the boil. Add the broccoli florets and cook for 1-2 minutes, or until they have softened slightly but are still firm. Drain.
Simmer the fish stock, lemongrass, chopped onion and tomato purée in a separate saucepan over a medium heat.
Add the prawn heads to the stock. Simmer for 5-7 minutes, then remove from the heat, strain, and set the stock aside. Discard the prawn heads.
For the prawns and broccoli, heat the oil over a medium heat, then fry the onion, garlic and chilli for 3-4 minutes to soften.
Add the prawns and fry until they are pink and cooked through. Then add the toasted cashew nuts and the broccoli florets.
Pour 150ml/5fl oz of the reserved fish stock into the pan, mix well, then stir in the lime juice. Heat through for 2-3 minutes, then stir in the coriander and chives.
To serve, spoon the broccoli and broth into a bowl and arrange the prawns on top of the dish.