Lightly coat an unheated large nonstick skillet with nonstick cooking spray. Preheat over medium heat. Add onion halves to skillet, cut sides down. Cook for 5 to 10 minutes or until onion is charred. Remove from heat and set onion in skillet aside.
Strain liquid from turkey roasting pan through a fine-mesh sieve into a 4-cup heatproof glass measure; discard solids. Skim off and discard all of the fat from the liquid. Add enough canned chicken broth to remaining liquid to measure 3 cups total liquid.
Add broth mixture, rosemary, sage, thyme, bay leaf, and black peppercorns to onion halves in skillet. Bring to boiling; reduce heat. Simmer, uncovered, for 7 minutes. Strain broth mixture, discarding solids; return broth mixture to skillet.
In a small bowl, combine ½ cup reduced-sodium chicken broth, the flour, salt, and ground black pepper, whisking until smooth. Add to broth in skillet. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more.