In a large pot or Dutch oven, brown the Italian sausage over medium-high heat, breaking it up with a spoon. Drain off any excess grease.
Add the chopped onion to the pot and cook until softened, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant.
Add the diced sweet potatoes, chicken broth, dried thyme, and cayenne pepper (if using) to the pot. Bring to a boil, then reduce heat and simmer, covered, for 20-25 minutes, or until the sweet potatoes are tender.
Stir in the heavy cream and cook for another 2-3 minutes, until heated through. Do not boil.
Season the soup with salt and black pepper to taste.
Ladle the soup into bowls and garnish with fresh chopped parsley.
Notes
This soup can be made ahead of time and reheats well. For a thicker soup, you can mash some of the sweet potato against the side of the pot.