Melt butter in large saucepan over medium heat.
Add onions and sauté until they are tender, about 5 minutes.
Add broth, increase heat to high, and simmer.
Remove pot from heat, add sweet potatoes, and let stand covered for 20 minutes.
Add brown sugar, thyme, vinegar, 1 teaspoons salt, and ½ teaspoon pepper.
Stir to combine.
Bring to simmer over high heat.
Reduce heat to medium low, cover, and cook until potatoes are very soft, about 10 minutes.
Using an immersion blender or working in batches with a stand mixer, puree soup until smooth. If needed, return soup right back into your pot.
Bring your pureed soup to a simmer over medium heat.
Add more broth if necessary to reach your desired consistency.
Season with salt and pepper to taste.
Serve with a tablespoon of sour cream of whipping cream and green onions if desired. Enjoy!