Heat oven to 425F.
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.
When soup comes to a boil, add pasta and cook 2-3 minutes then add white beans and reserved sausage. Continue to cook pasta to al dente, 4-5 minutes more. Turn off heat and wilt in greens, if using.
Serve in shallow bowls then top fennel fronds and cheese, if using.