Remove vegetables from roasting pan and discard. Pour pan juices into a glass measuring cup or gravy separator, and let stand until separated, 10 minutes. Pour off fat.
Place roasting pan over 2 burners on medium heat. Add wine, and simmer, stirring and scraping pan constantly for 2 minutes. Add the reserved juices and 4 cups stock, and bring to simmer.
Meanwhile, whisk together remaining 1/2 cups stock and the flour in a bowl until smooth. Whisking constantly, slowly add flour mixture to pan. Simmer until gravy is reduced by half and coats the back of a spoon, 10 to 12 minutes.
Remove from heat, add butter, and swirl pan until it has melted. Pour through a fine sieve, discarding solids. Serve immediately or keep warm over low heat. (Gravy can be refrigerated in an airtight container for up to 1 week.)