Preheat oven to 350°F. Roast beets until tender when pierced with fork, about 1 hour. Cool. Peel beets. Cut into cubes.
Melt butter with oil in heavy medium saucepan over medium-high heat. Add leeks, onion, carrots, fennel, and celery and cook until beginning to brown, stirring frequently, about 13 minutes. Stir in spices and beets. Cook until vegetables begin to stick to bottom of pot, stirring frequently, about 7 minutes. Add boullion and bay leaf. Bring to boil. Reduce heat to low, cover, and simmer until vegetables are very tender, about 25 minutes. Remove bay leaf and cool soup slightly. Working in batches, puree soup in blender with cream. Season to taste with salt and pepper. (Can be made 1 day ahead. Cool slightly, cover, and refrigerate.)
Gently rewarm soup (do not boil). Divide between bowls. Garnish each with 1 tablespoon crème fraîche. Top with a sprig of thyme.