In a medium saucepan, combine the onion and fat over medium heat and cook until the onions are completely tender and just beginning to brown (if you want to brown them, go ahead), 5 or 10 minutes.
If you want a thin gravy, add 4 tablespoons of flour, for a thicker gravy, add 6 tablespoons. Continue to cook the onions with the flour till the flour lightly browns and loses its raw smell.
Turn the burner to high and pour in the stock, whisking continuously as you do.
Keep cooking until the gravy comes to a boil and thickens. Season with salt to taste. Skim and discard any foam that collects on the side of the pan. Add a few drops of lemon juice or white wine vinegar.
Taste. Does it need more acidity or salt? Add it. Would you like to put pepper in it? Go ahead, it's your gravy. Fan of giblets? If you roasted the gizzard and heart with the turkey, chop those up and add those. Too thick? Add a little more stock from the cool cooking method I posted on Monday. Too thin? Either make a quick roux of fat and flour and add that or a slurry of cornstarch and water. When it's perfect, it can keep warm for ages, covered, while you work on the rest of dinner. Serve piping hot.