Meanwhile, set the medium skillet over high heat. Add 1/2 cup of the wine and bring to a boil, scraping up any browned bits. Pour the wine into the large skillet and add the garlic and thyme. Set the large skillet over high heat, add the remaining 1/2 cup of wine and boil until reduced by one-third, about 3 minutes. Add the chicken stock and boil until reduced to 1/2 cup, about 8 minutes. Remove the skillet from the heat and discard the garlic and thyme sprig. Whisk in the butter, 1 piece at a time. Season with salt and pepper and strain the sauce into a warmed gravy boat.