I love using vegetables to thicken a sauce instead of a more traditional choice like flour or cornstarch. The flavor of the garlic and onions here adds richness without being heavy. I think it is also tasty with the flavor of the turkey drippings. Additionally, the garlic cloves that are left somewhat whole in the gravy add texture. All that’s missing are some potatoes and a platter of carved turkey...
Arrange the garlic halves and sliced onions side by side in a single layer of foil with some foil to spare. Drizzle with the olive oil and season generously with salt. Wrap the foil over into a little package and place in the oven. Roast until completely tender when pierced with the tip of a knife, 45 to 50 minutes. Carefully open the foil and let cool. Squeeze the garlic out of their skins, using a fork or a knife if necessary to pry them out. Discard the skins.
In a small saucepan over medium-high heat, simmer the sherry until reduced to a couple of tablespoons, about 10 minutes.
Once the turkey has been transferred from the roasting pan to a cutting board to let rest, put the roasting pan over 2 burners and heat over medium high. Add the chicken broth. Using a whisk, scrape the browned bits from the bottom of the pan. Add the garlic and onions, mustard, vinegar, salt to taste, pepper and reduced sherry. Let the flavors infuse for a couple of minutes. Transfer most of the onions and garlic and some of the liquid from the roasting pan, about 1 1/2 cups, to a blender and blend until smooth. Return the puree to the roasting pan and bring to a boil, Cook, constantly whisking, until reduced to desired consistency. Adjust the seasonings.