In a large bowl, mix the flour and salt together. Then add the natural yogurt and olive oil. Knead for a few seconds until the dough has an even consistency.
Preheat the oven to 180°C/350°F (conventional oven).
Sprinkle some flour on the worktop and then use a rolling pin to roll out the dough into a large circle (approximately 3 mm/⅛ inch thick).
Place the pastry circle on a sheet of baking paper and spread with a thin layer of grain mustard, but stop short of the edges.
Wash and finely cut the tomatoes. Arrange on the layer of grain mustard.
Finally, fold over the edges of the pie towards the center. Sprinkle with the basil and bake in the oven for 35 to 40 minutes, while keeping a close eye on the pie as it cooks.
Once cooked, season and sprinkle with some crumbled feta.