Melt the butter in a 2 quart saucepan. Tilt the pan to coat the bottom and sides. Chop the fish trimmings and bones. Add the onion, parsley and peppercorns. Add a 1/2 cup of water. Cover the pan and simmer for 20 minutes. Add the remaining 2 1/2 cups of water, wine and salt. Cover and simmer for 30 minutes or longer. Strain the stock. Use as directed in recipes calling for fish stock.