Roast turkey legs in the oven at 325F for 1.5 to 2 hours, until done: meat thermometer will show 180F.  Remove meat from legs, reserve it for another use.  Place bare bones back in oven; roast for a further half-hour.

Make turkey stock:

Break turkey leg bones once each, and place in a large stock pot with celery, carrot, onion, garlic, peppercorns, and herb stems (for this, use any leftover stems of parsley, thyme, rosemary, etc, that you may have left after stripping the leaves and using for other purposes).  Fill with enough water to cover the bones and vegetables.  Bring to a boil; simmer on low at least three hours, until the bones break easily.

Strain the solids out of the stock; discard.  Boil the remaining liquid (your stock) down to 1 liter / 1 quart / 4 cups.

Enhance the stock:

In a deep 12" skillet or pot (something with at least a 3 qt/L capacity), heat oil over medium-high heat until hot.  Add turkey wings and cook 10-15 minutes, or until golden on all sides.

Add onion, carrots, celery, and garlic; cook 8-10 minutes or until turkey wings and vegetables are browned, stirring frequently. 

Transfer turkey and vegetables to a large bowl.  Add vinegar and 1/2 cup reserved turkey stock (or 1/2 cup dry white wine instead of both the vinegar and stock) to skillet, and stir until browned bits are loosened.

Return turkey and vegetables to skillet.  Stir in rest of your reserved turkey stock, thyme, and water; heat to boiling over high heat.  Reduce heat to medium-low; simmer, uncovered, 45 minutes.

Strain into a 2 L / 2 qt / 8 cup measuring jug or large bowl; discard solids (or save the meat off the wings if you like).  Place broth in container(s) that will fit in your fridge; refrigerate overnight.

Make the gravy:

The fat will have risen to the top, and both the fat and the stock will have congealed.  With a spoon, remove the fat from atop the stock.  Place 1/2 cup into a 2-quart/L saucepan.  If you have more fat, set aside for another use; if you don't have 1/2 cup fat, make up the missing amount with butter.

Add flour to the fat in the saucepan; cook, stirring over medium heat until flour turns golden brown.  Gradually whisk in reserved stock and cook until gravy boils and thickens slightly, stirring constantly.  Taste and adjust seasonings as desired.

Pour gravy into a 2-qt container or medium bowl; cover and refrigerate.

At serving time:

Reheat gravy and add pan drippings from the roast turkey if you like.

Gravy can be reheated in microwave prior to serving.