Bring the vegetable broth and 1 cup water to a simmer in a large pot set over medium-high heat. Add the broccoli and cook, stirring occasionally, until thawed but still bright green, about 5 minutes. Stir in the butter, thyme, 1-1/2 teaspoons salt, and 3/4 teaspoon pepper.
Add 1 cup of the cream, increase the heat and bring to a boil. As soon as it reaches a boil, reduce to a simmer and cook until the broccoli is quite tender, about 5 minutes.
Meanwhile, in a small bowl, whisk together the sour cream and remaining 1 tablespoon of cream and 1 tablespoon of lemon juice. Set aside.
Working in batches, if necessary, in a blender combine the broccoli and broth, cover, and blend on high speed until smooth; pour the soup into a bowl. Stir in the remaining 1 tablespoon lemon juice, and salt and pepper to taste.
Return the soup to the pot, bring to a simmer over medium heat, and heat until hot, thinning with water to change the consistency if desired.
Ladle into 4 bowls, drizzle 1-1/2 tablespoons of the lemon crème over each soup portion, and serve.