Assemble all your ingredients before you start — this comes together quickly once the pan is hot.
Make the Roux
Make a roux with the melted butter and flour, and set it aside.
This can be made ahead and held at room temperature for an hour or two if that helps your timing.
Bring the Stock to a Boil
In a saucepan, bring the chicken or turkey stock, thyme sprig, and pan drippings to a boil, then reduce to a simmer.
Whisk in the Roux
Whisk the reserved roux into the simmering stock, working out any lumps as you go.
Cook to Thicken
Cook over low heat for about 10 minutes, until the gravy reaches the desired thickness.
Season and Strain
Season with salt and pepper to taste. Strain to remove the thyme sprig and any stray bits from the pan drippings, then serve.
Notes
Most sauce recipes say to strain at this point, and it is better for presentation, but I honestly can't tell you I do it all the time. Just be sure to get the sprig of thyme out of the gravy boat.