Mix together yoghurt and salt in a large bowl until smooth. Gradually add the flour and stir with a butter knife until mixture forms a stiff dough. Knead on a lightly floured surface, until the dough is soft and slightly sticky. Place into a clean bowl, cover and stand for 30 minutes. Divide dough into 4 round pieces. Roll each piece into a 20-25 cm circle. Sprinkle spinach and feta over half the circle and season. Fold dough over to form a half circle and press edges with a fork. Repeat as necessary. Preheat barbeque plate or frying pan. Bush one side of each gozleme with olive oil and cook until the base is golden. Gently turn gozleme and cook until golden. Serve with lemon wedges and melted butter if desired.