Set the roasting pan over two burners on medium-high heat. Add cider and vinegar; bring to a boil and cook, scraping up the browned bits from the pan, until the liquid is reduced by about half, 6 to 8 minutes. Add the remaining 3 1/2 cups stock (or broth). Increase heat to high; return to a boil, whisking often. Boil until the liquid is reduced by about half, 8 to 12 minutes.