Preheat oven to 425 degrees F. Toss together parsnips, carrots, 2 tablespoons olive oil, red pepper flakes, thyme, salt and pepper on a baking sheet. Bake until lightly caramelized, about 30 minutes, depending on how large your vegetables were chopped. Remove the thyme stems.
Warm the remaining tablespoon of oil in a medium pot over medium heat. Add the shallot and cook for a few minutes, until translucent. Add the garlic and cook another minute. Add the roasted vegetables and stock. Puree with an immersion blender (or a regular blender, in batches if necessary), adding more stock as needed to get the consistency you want. Season with salt and pepper to taste.
For the Lemon Ginger Cream: Combine all ingredients in a small bowl and whisk until well combined. Garnish the soup generously with the lemon ginger cream.