Preheat oven to 425F and lightly grease a large sheet pan. Place sweet potatoes on prepared sheet pan and roast at 425F 55-60 minutes until sweet potatoes are tender when pierced with a fork.
Once sweet potatoes are cool enough to handle, scoop out interior and place in a bowl, discarding skins (or add a few to soup for some additional texture)
In a medium pot or Dutch oven over medium heat, cook bacon until crispy. Drain on paper towels and set aside.
In same pot over medium-high heat, cook onion in bacon drippings until tender. Add garlic and cook 1-2 minutes until fragrant.
Add baked sweet potatoes, 3 cups chicken broth, and milk to pot and bring to a simmer over medium-high heat.
Transfer soup to a blender or use an immersion blender to puree soup until smooth. Add back to pot and stir in sour cream and additional chicken broth as needed to thin soup. Season soup with salt and pepper to taste.
Ladle soup into bowls and top with additional sour cream, crispy bacon bits, green onion or parsley, and cheddar on top. Enjoy!