Place a saucepan over a medium heat and cook the onions and garlic for 10 minutes or until soft. Stir in the anchovy fillet until it has dissolved then add the lemon grass, lime leaves and ginger and cook for 1 minute. Add the aubergines, coriander stalks, coconut milk and fish sauce to taste. Simmer for 20 minutes. Stir in the spinach, coriander and basil leaves, reserving a little to garnish.
Meanwhile, melt the butter in a large saucepan over a medium heat, stir in the stock and prawn shells and bring to a simmer. Turn the heat to its lowest setting and place the fish fillets into the liquid, skin side up. Cook gently for 6-8 minutes adding the prawns halfway through.
To serve, spoon the coconut mixture between two large shallow bowls, scatter over the chives, prawns and break over the poached trout. Garnish with more coriander and basil leaves and a squeeze lime.