The shells are flavourful and make a great stock to use in gumbo, paella or bouillabaisse. The ratio is for the heads, shells and tails from 450 grams (1 pound) of prawns using four cups of water.  For a quick stock, fry the prawn heads and shells with some garlic and fresh thyme (optional) from the garden until they begin to colour. I then add water and boil for 30 minutes. I drain and cool the stock and portion it into containers in the fridge until I am ready to use.  For a more robust stock, I will add a roughly chopped onion when I am frying the shells and add a carrot, a small celery stock, 3-4 sprigs of parsley, wine and peppercorns with five cups of water.