Make Ahead: Pour cooled gravy into quart-size zip-top plastic freezer bags; seal and lay bags flat on a baking sheet. Place in freezer. Once frozen, the bags are easily stackable. Thaw three to four days in the fridge before reheating on the stovetop.
Melt butter in a medium saucepan over medium heat. Add onions, and cook, stirring occasionally, 5 minutes. Sprinkle flour over onions, stirring constantly, and cook, stirring constantly, 2 minutes or until flour is golden brown.
Gradually whisk in 4 cups stock. Cook, stirring often, 5 minutes or just until mixture comes to a boil and is smooth and thick. (If mixture is too thick, add up to 1 more cup stock, 1/4 cup at a time, until desired consistency is reached.)
Season with salt and pepper. Cool completely (about 30 minutes). Cover and store in refrigerator up to 3 days.
Note: To add turkey flavor to gravy, skim fat from drippings in roasting pan, and stir 2 to 3 Tbsp. water into drippings. Bring to a simmer over medium-high heat, stirring and scraping to loosen browned bits from bottom of pan. Pour drippings mixture into warm gravy, and stir gently.
We tested with Swanson Natural Goodness 33% Less Sodium Chicken Broth.