Sprinkle the turkey pieces with salt & pepper. In a large Dutch oven or large saucepan, heat oil over MED-HIGH heat. Brown the turkey pieces 3-5 minutes on each side then add the carrots, celery and onion. Cook another 5 minutes, turning once, until vegetables have begun to caramelize and have some golden brown color. Turn heat to LOW, cover and cook for 15-20 minutes. Stir every 5 minutes.
Scrape the pan, while slowly adding the sherry or wine, to deglaze the pan. Continue scraping the pan and add the chicken broth, stir continuously.
Add herbs, turn heat to HIGH and bring broth to a boil. Reduce the heat to LOW and simmer, uncovered, for 30 minutes. Skim excess foam off the surface as needed.
Pour broth through a fine-mesh strainer over a large heat-resistant bowl. Shred the turkey meat from the bones, and (if desired) reserve ¼-½ the meat for the gravy. Freeze the remaining meat, in a freezer bag, for later use. Pour the broth back into the large saucepan, and bring it to a low boil.
To make the roux, melt butter in a medium saucepan over MED-LOW heat and whisk in flour. Cook, stirring continuously, until the flour mixture is fragrant and nutty brown, 10-15 minutes.
While whisking vigorously, slowly add 2 cups of the hot broth to the roux (flour/butter mixture). Whisk until the mixture is smooth and there are no lumps. Whisk the roux mixture into the simmering broth in the large saucepan.
If making gravy with the reserved turkey meat, add it to the gravy now.
Season the gravy to taste, and bring to a boil. Reduce the heat to low, stirring often, and keep at a simmer until gravy has thickened or about 20-30 minutes.
Cover and set aside to cool to room temperature. You now have turkey gravy. Cover & refrigerate or freeze until ready to use.