For the dough, put all the dough ingredients in a bowl and knead with an electric mixer (dough hook) into a smooth dough. For the topping, halve the leeks lengthwise, trim and cut into thin half-rings. Sauté the leeks in oil, season with salt and pepper and simmer until tender, about 10 minutes.
Roll out the dough on a floured surface and line the quiche pan with the dough. Peel the apple, quarter, core and cut into slices. Mix apple with the leeks. Rinse the tarragon, shake dry, pluck off the leaves and chop finely. Mix yogurt, milk, egg and potato starch with the leek mixture. Spread on the dough and sprinkle with sunflower seeds. Bake in preheated oven at 200°C (approximately 400°F) about 40 minutes. Serve lukewarm.