Combine stock, turnips, potatoes, onion, garlic, thyme sprigs, salt, and pepper in a 6-quart slow cooker. Cover and cook on LOW until vegetables are very soft, about6 hours. Remove and discard thyme sprigs. Add cream, and process soup using an immersion blender until smooth. (Or transfer to a blender, and remove center piece of blender lid to allow steam to escape. Secure lid on blender, and place a clean towel over opening in lid. Process until smooth.)
Just before serving, cook bacon in a non-stick skillet over medium until crisp, 6 to8 minutes. Drain bacon on paper towels, reserving drippings in pan. Add turnip greens to skillet; increase heat to medium-high, and cook, stirring occasionally, until wilted and beginning to brown just around edges, about 3 minutes. Crumble bacon. Serve soup topped with crumbled bacon and wilted greens.