Transfer the reserved vegetables and pan juices to a large skillet. Cook over medium-high heat, stirring occasionally, until the vegetables darken and the liquid thickens, 4 to 6 minutes.
Add the flour and cook, stirring, for 1 minute. Whisk in the broth and cook until thickened, 10 to 12 minutes.
Strain the gravy through a fine-mesh sieve. Serve warm.