1. Quickly knead flour, pieces of butter, 1 egg and 1 pinch of salt into a smooth dough. Roll out the dough between two layers of cling film (approx. 32 cm). Line a greased tart mold (26 cm) with it. Prick the bottom of the dough several times with a fork. Refrigerate for approx. 30 minutes.
2. Peel and slice sweet potatoes into 2mm thick slices. Wash spring onions and cut them into rings. Lay aside some spring onion garnish. Steam sweet potatoes and spring onions in hot oil for approx. 5 minutes. Season with salt, pepper and thyme.
3. For the icing, thoroughly whisk crème fraiche and 3 eggs. Season with salt, pepper and nutmeg.
4. Cover the pastry base with baking paper and fill with dried pulses. Bake in the preheated oven at 200 °C (convection oven: 180 °C) on the lowest rack for approx. 15 minutes. Remove baking paper and pulses. Place sweet potatoes and spring onions on the bottom of the dough. Pour the icing over it. Bake on the lowest rack for another 30 minutes.
5. For the tapenade, chop olives, capers, anchovies and peeled garlic cloves very finely. Stir in olive oil. Sprinkle the finished tart with spring onions and serve with the tapenade.
Preparation time: approx. 1 1/2 hours