In a mixing bowl, combine the flour and salt. Add the cubed butter and rub it into the flour until the mixture resembles breadcrumbs. Gradually add cold water, mixing until the dough comes together. Wrap in plastic wrap and refrigerate for at least 30 minutes.

In a skillet, heat olive oil over medium heat. Add the chopped onion and sauté until softened and translucent. Remove from heat and let cool slightly. Mix in the grated cheese and season with salt and pepper.

On a floured surface, roll out the pastry dough to about 1/8 inch thick. Cut out circles using a large cookie cutter or a bowl (approximately 6 inches in diameter).

Place a spoonful of the cheese and onion filling in the center of each pastry circle. Fold the pastry over to create a half-moon shape and press the edges together to seal. Use a fork to crimp the edges for a decorative touch.

Preheat your air fryer to 350°F (175°C).

Place the filled pastries in the air fryer basket, making sure they are not touching. Cook for 12-15 minutes or until golden brown and crispy.