Bake the sweet potatoes in aluminium foil for 45 min. at 180 °C circulating air until cooked through. Then leave to cool.
Remove the sweet potatoes from their skin and cut into pieces. Peel the mango and remove the core, cut into cubes and squeeze the lemons.
Blend all the ingredients into a smooth smoothie.
If you think the smoothie is too thick, dilute it with a bit of fruit juice. You can sweeten the smoothie slightly with sugar or some honey if required.