While turkey rests on cutting board, place roasting pan on stove across two burners. Bring pan juices to a simmer over medium-high. Cook, scraping up browned bits with a wooden spoon, until liquid thickens and holds a trail, 8 to 10 minutes.
Add flour and cook, stirring, until blended, about 1 minute. Gradually whisk in stock, then 1 cup water. Bring to a simmer and cook, stirring occasionally, until gravy thickens and no longer tastes floury, about 5 minutes (reduce heat if bottom of pan begins to scorch).
If desired, strain gravy through a fine-mesh sieve. Season with salt and pepper and keep warm. Whisk before serving (thin with water if necessary).