Process all the dough ingredients in a food processor and refrigerate for half an hour.

Cook the sweet potatoes in boiling water with a teaspoon of salt for 7 minutes until softened, then transfer to a bowl.

Preheat the oven to 355°F (180°C).

Grate the Gouda cheese and set aside. In another bowl, mix eggs, yogurt, cream cheese, whipping cream, pepper, and salt. Set this mixture aside.

Roll out the dough to a thickness of about 3 mm, cut a circle using a 24 cm diameter ring or a round springform pan of the same diameter. Roll out the remaining dough again, cut two strips to cover the edges of the pan, press well, and cut the remaining dough with a knife.

Sprinkle a handful of Gouda cheese at the bottom, add the potato chunks, and top with another handful of Gouda. Pour the filling mixture over and sprinkle the mozzarella balls on top. If there are leftover potato chunks, spread them too.

Bake for 75-90 minutes or until set.

Remove from the oven and let it cool for about half an hour. Do not remove the sides of the pan before cooling; the quiche needs to stabilize.