In a mixing bowl, combine the flour and salt. Add the cold butter and rub it into the flour until the mixture resembles breadcrumbs.

Gradually add cold water, mixing until a dough forms. Wrap in plastic wrap and refrigerate for 30 minutes.

In a saucepan, boil the diced potatoes until tender, about 15 minutes. Drain and set aside.

In a skillet, melt butter over medium heat. Add chopped onion and sauté until translucent, about 5 minutes.

Combine the potatoes, sautéed onions, grated cheese, salt, and pepper in a bowl. Mix well.

Preheat the oven to 400°F (200°C). Roll out the pastry dough on a floured surface to about 1/8 inch thick.

Cut out circles of dough, about 6 inches in diameter. Place a spoonful of filling on one half of each circle.

Fold the dough over to create a half-moon shape and seal the edges by crimping with a fork.

Place the pasties on a baking sheet lined with parchment paper. Brush the tops with a little milk or egg wash.

Bake in the preheated oven for 25-30 minutes or until golden brown.

Remove from oven and let cool slightly before serving.