Serves 6 For the stock: 1 pound prawn heads and shells (about 8 cups), rinsed 4 quarts of cold water ½ medium onion, sliced 1 rib celery, cut crosswise into ¼ inch pieces 1 carrot, roughly chopped Kosher salt For the risotto 7-8 cups prawn stock 1 ½ -2 pounds…