In a large heavy saucepan heat the olive oil over very low heat. Add the celery, onion, carrots, leek and thyme. Cook for about 30 minutes, until very soft and brown. Stir occasionally and keep the heat low to prevent the vegetables from burning. Add the bay leaves, peppercorns, and stock, increase the heat and bring the liquid to a boil. Immediately reduce the heat to very low and simmer, covered for 15 to 20 minutes. Strain the stock into a clean pan, pressing down on the solids to extract all their flavor. Discard the solids. Add the large bunch of thyme to the broth and simmer actively for 15 minutes. Strain again and add the roasted garlic cloves, cream, salt and pepper. Taste for seasoning and serve in heated cream soup bowls. Sprinkle the toasted croutes with a little Parmesan. Top each bowl with a toasted croute and a sprig of thyme.
Preheat the oven to 350 degrees. With a sharp knife, slice off the very top of the garlic, just to expose most of the cloves (this will make them easier to squeeze out after cooking). Make a doubled layer of aluminum foil about 6 by 6 inches square. Place the garlic in the center and bring up the sides a little to form a cup. Drizzle over the olive oil and add the oregano, salt and pepper. Bring the sides all the way up and twist together to make a firm seal. Place in the oven and roast for 20 to 25 minutes. Open the top so the garlic will stop cooking and set aside until cool enough to handle. Squeeze the garlic cloves out of their skins and discard the skins.