Peel the sweet potatoes and waxy potatoes and cut into sticks (approximate french fries size). Transfer to a large bowl.
Add the olive oil, salt, polenta and cornstarch to the potatoes and mix well with your hands.
Place the fries evenly and preferably without overlapping each other on a baking sheet lined with baking paper and bake in the upper third of the oven for about 20 minutes. Turn the fries over and bake for another 10 minutes until well browned.