Pour drippings from pan into a bowl; skim fat and discard onion. Set pan across two burners over medium-high. Add wine.
Cook, stirring and scraping up browned bits with a wooden spoon, until mixture thickens and holds a trail, 4 minutes.
Add flour and reserved drippings and whisk to combine.
Continue whisking until flour is fully cooked, 2 to 3 minutes.
Slowly whisk in broth, then 1 cup water. Cook, whisking, until gravy thickens, 15 minutes.
If desired, strain gravy through a fine-mesh sieve. Season with salt and pepper. Keep warm (thin with water if desired).