Coconut fish curry

Name: Coconut fish curry
Ingredients: 1 tbsp vegetable oil; 1 onion finely chopped; thumb-sized piece ginger finely grated; 3 garlic cloves crushed; 1 tsp shrimp paste; 1 small red chilli shredded (deseeded if you don't like it too hot); 2 lemongrass stalks, split, then bruised with a rolling pin; 1 heaped tbsp medium curry powder; 1 heaped tbsp light muscovado sugar; small bunch coriander stems finely chopped; 400g can coconut milk; 450g skinless hake fillets, cut into rectangles, roughly credit card size; 220g pack frozen raw whole prawns; 1 lime halved; cooked rice to serve