Preheat oven to 450 degrees. Cut onion into 8 wedges (keep root end intact to hold layers together). Toss onion, carrots, endive, oil, salt, and pepper in a medium bowl. Transfer to a rimmed baking sheet, and spread in a single layer. Roast vegetables, turning occasionally, until edges are deep golden brown, about 30 minutes.
Cut off root end from onion. Transfer all vegetables to a large saucepan, and add bay leaf. Add enough stock to just cover (about 2 cups). Bring to a simmer, and cook until carrots are very soft, about 30 minutes. Let cool slightly, and discard bay leaf. Puree vegetables and stock in a blender until smooth (work in batches, if necessary, to avoid filling blender more than halfway).
Transfer puree to a clean pan; place over low heat. Stir in cream; add stock to thin soup to desired consistency. Season with salt and pepper; stir in ginger. If desired, pipe creme fraiche onto each serving, or place dollop on top.