Prawn risotto

Ingredients

• 500g/1lb 2oz prawns in shells, peeled and black vein removed, shells and heads reserved (or 350g/12oz peeled prawns)
• 1.3 litres/2¼ pints water (or light fish stock)
• 1 small brown onion, finely chopped
• 1 stick celery, finely chopped
• 1 small fennel bulb, finely chopped (optional)
• pinch salt
• 5 tbsp extra virgin olive oil, plus extra to serve
• 320g/11½oz risotto rice
• 100ml/3½fl oz dry white wine
• 1 lemon, zest only
• 1 tbsp roughly chopped fresh parsley (optional), to serve