1. In a large saucepan over medium heat, melt butter. Stir in shallots, turn heat down to low, and saute until they have softened. Stir in potatoes and carrots and add stock. Bring mixture to a boil, and then cover, turn heat down to low, and simmer for about 10 minutes, until the carrots and potatoes are tender.
2. Remove from heat. Stir in parsley, basil, cilantro and tarragon. Let cool for about 10 minutes. Puree soup in a blender; you’ll probably need to work in batches. Or you can use an immersion blender if you have one. Pour the pureed soup back into the saucepan. Stir in lemon juice and season with salt and pepper to taste.
3. Transfer soup to serving bowls. Top with a dollop of crème fraîche, if desired.