Heat your oven to 400 degrees Fahrenheit. Slice the top off the head of garlic so the tips of the cloves are exposed. Set it cut side up on a small piece of foil, drizzle with the olive oil, and wrap it tightly. Roast for 45 minutes until the cloves are deeply golden and feel completely soft when you press on the outside of the packet. Let it cool until you can handle it, then squeeze the softened garlic out of the paper skins into a small bowl. Mash it to a rough paste with a fork.

While the garlic roasts, warm your stock in a small saucepan over low heat. You want it hot when it goes into the roux so the gravy comes together smoothly without lumps. Keep it on low until you are ready for it.

Melt the butter in a medium saucepan over medium heat. Once it is foamy but not browning, whisk in the flour all at once. Keep whisking for about 2 minutes. The roux should smell slightly nutty and turn a pale golden color. This step cooks out the raw flour taste, so do not rush it.

Add the mashed roasted garlic to the roux and whisk to combine. Now start adding the warm stock, about 1/2 cup at a time, whisking constantly after each addition until smooth before adding more. Once all the stock is in, the gravy should look glossy and slightly thick.

Bring the gravy to a gentle simmer over medium-low heat and cook, stirring frequently, for about 8 to 10 minutes until it coats the back of a spoon and holds a line when you drag your finger across it. Stir in the Worcestershire sauce, salt, and pepper. Taste and adjust seasoning.

If any lumps have formed, pour the gravy through a fine mesh strainer into a clean container while it is still warm. Let it cool to room temperature, then cover and refrigerate overnight or for up to 3 days.

On Thanksgiving Day, reheat the gravy in a saucepan over low heat, whisking as it warms. If it is thicker than you want, whisk in warm broth a splash at a time until it loosens up. Once the turkey comes out of the oven, stir in 2 tablespoons of the pan drippings. Taste one more time for salt before serving.