Heat oven to 425°F.
Cut the end off the head of garlic and drizzle with EVOO and season with salt and pepper. Wrap it in foil and roast to soft, 40-45 minutes. Squeeze roasted garlic cloves into a small bowl and mash with a fork.
Heat EVOO in a soup pot over medium-high heat. Add sausage, if using, and brown and crumble into small bits. Remove and drain, leaving a tablespoon and a half or so of drippings to cook in. Add fennel and onions to the pot and season with salt and pepper. Cook 10 minutes to soften, then add mashed garlic, herbs and stock. Bring to a boil -- add 1-2 cups water if you like a thin, brothy soup.