Coarsely grind the sheep's cheese. Stir 5 eggs until foamy, then mix with the yogurt. Add oil and the grated cheese to the mixture. Season with salt.
Preheat the oven to 170°C (338°F).
Brush a baking dish with oil. Layer the yufka pastry sheets and egg-cheese mixture alternately (starting with pastry, then filling, continuing to alternate) until the egg mixture is used up, ending with a yufka layer on top.
Mix the remaining egg with the sparkling mineral water. Pour this mixture over the banitza. Pierce the banitza in several places with a fork so the liquid penetrates. Sprinkle black cumin on top.
Bake for 45 minutes at 170°C (338°F) until the top is golden and set.
Serve warm or at room temperature, optionally with yogurt on the side.