Mix together the milk, quark, 4 tbsp oil and 1/2 tsp salt.
Add the flour and baking powder and mix to a smooth dough.
Peel the potatoes and grate or plane into very fine slices.
Wash the sage, shake dry and roughly chop the leaves. Peel and very thinly slice the onion.
Mix the potatoes, onions and sage with the remaining oil and season with salt and pepper.
Divide the dough into 4 portions and roll out each one to a 23cm round.
Place 2 pizzas on each baking tray lined with baking paper.
Cover 2 pizzas with half the potato-onion mixture and bake in the preheated oven for 15-20 minutes at 220°C (200°C fan, gas 7).
Repeat with the remaining two pizzas.