Roasting the garlic and cauliflower lends a sweet, earthy note to this thick soup.
Position a rack in the lower third of the oven and preheat to 400 degrees F.
Place the cauliflower florets on a rimmed baking sheet and toss with 2 tablespoons of olive oil. Season liberally with salt and pepper. Set aside while you prepare the garlic.
Remove the loose, papery skins from the garlic head. With a sharp knife, cut off the top 1/3-inch of the head to expose the cloves. Rub the cut end with a small amount of olive oil. Wrap the garlic in a piece of aluminum foil and place the packet on the baking sheet with the cauliflower.
Transfer the baking sheet to the oven and roast the cauliflower and garlic, turning the florets every 10 minutes with a spatula, until it's golden brown and tender, about 25 to 35 minutes. Remove the tray from the oven and let cool slightly. Don't open the garlic packet.
Meanwhile, heat the remaining 1 tablespoon of oil in a large saucepan over medium heat. Add the leeks and sauté, stirring frequently, until the leeks soften, about 10 minutes. Don't allow the leeks to brown. Add the wine and cook until most of the liquid evaporates, 3 to 5 minutes.
Add the roasted cauliflower to the leeks, and then add the chicken stock. Raise the heat to medium-high and bring the mixture to the start of a simmer.
Remove the foil from the head of roasted garlic. Squeeze the bulb to extract the soft garlic pulp. Add as much as you like to the saucepan.
Cook the soup, covered, for 15 minutes to allow the flavors to meld. Remove the pan from the heat and allow the soup to cool for 10 minutes.
Transfer the soup to a food processor and purée until smooth. It may be necessary to do this in batches, depending on the size of your food processor. Return the purée to the saucepan, and add the half & half and fresh thyme. Reheat slowly over medium heat. Serve the soup hot, drizzled with some white truffle oil, if you like.