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 minutes.
Stir in the minced garlic and cook for 1 minute more until fragrant.
Add the cubed sweet potatoes, chicken broth, dried thyme, red pepper flakes (if using), salt, and black pepper. Bring to a boil, then reduce heat and simmer for 15-20 minutes, or until the sweet potatoes are tender.
Stir in the chopped kale and cook for another 5 minutes, or until the kale is wilted.
Taste and adjust seasonings as needed. Serve hot.
Notes
This soup is delicious on its own or served with crusty bread.