Heat half of the butter in a soup pot over medium heat. Add onions and cook 5 minutes. Add garlic and cook for another 3 minutes.
Add all remaining ingredients to the pot including the rest of the butter. Bring to a simmer and cook for 15-20 more minutes, until everything is cooked through and sweet potato is fork tender.
Add soup to blender and blend until smooth. Alternatively you can use an immersion blender.
Divide out soup and serve with crusty bread or a grilled cheese.
Notes
*I used 4 cups and the soup came out pretty thick, which I liked. If you want a thinner soup add another cup of broth or even water.