Preheat the oven to 400 F/ 200 C/ gas mark 6. Place the sweet potato on a baking sheet and bake for about 30 minutes or until fully cooked and tender (check using a fork). Remove from the oven, leave to cool, then scoop out the flesh and place in the blender.
Cut both ends of the clementine, place on a cutting board and cut the rind off by slicing downwards all the way around the fruit. Add the fruit into the blender along with the mango and lemon juice. Puree until velvety smooth. Serve chilled.
Sweet potato: I recommend using a long, not very round sweet potato. The slimmer the potato the less time it will need in the oven. You can use either one larger potato or 2 small ones.
Make ahead: Place your sweet potato on a baking sheet and pop in the oven for about 30-40 minutes or until tender (check with a fork). Remove from the oven and leave to cool, then peel and make the smoothie. If you bake the sweet potato the night before leave the skin on until you are ready to make the recipe.
Mango: Use fresh or frozen.
Tangerines: These can be swapped for clementines or 1 large orange.
Storing: Refrigerate leftover smoothie in a bottle or jar for up to 2 days.