Place a large pot over high heat. Add onions, mixing constantly until golden without lowering the heat, 5 to 10 minutes. Add butternut squash and fennel. Add just enough water to stir vegetables without covering. Bring to a boil. Season with sucralose, cinnamon, salt, and pepper and reduce heat. Simmer soup until tender and thick, about 1 hour.
Blend soup using a stick blender until mostly smooth with a few chunks.