Use leftover baked sweet potatoes for this recipe or peel and dice enough sweet potato to make about 2 cups worth.
Add the sweet potato and cauliflower to a large soup pot and add the chicken broth. Simmer about 15 minutes or until the cauliflower is super soft. If you used raw sweet potato, make sure it is super-soft, too.
Meanwhile, clean the leeks and slice them thinly.
In a medium-size skillet, pan-fry the bacon until it is crisp. Set the bacon aside to use as a garnish. Remove all but about 2 tablespoons of bacon fat.
Sautee the leeks in the bacon fat in the same skillet, and when they're soft, add them to the cauliflower and sweet potato. Add the roasted garlic and thyme to the soup pot. Simmer an additional 5 minutes.
Use an immersion blender or blender to puree the soup until it is smooth and velvety.
Return the pureed soup back to the soup pot and heat it through until it is barely simmering. Add the cream and turn off the heat. Mix well to combine.
Temper the sour cream by adding a few spoonfuls of hot soup to the cold sour cream to bring the temperature up, then whisk the sour cream into the soup.
Season with a pinch of cayenne and salt and pepper to taste.
Serve with reserved bacon and a small dollop of sour cream.