In a large bowl, whisk together flour and salt until combined. Cut in the butter until the mixture resembles coarse crumbs.

Gradually add ice-cold water, mixing gently until a soft dough forms. Wrap in plastic and refrigerate for 30 minutes.

Boil water, add chopped potatoes, and cook until tender, about 15-20 minutes. Drain and mash lightly.

Melt butter in a skillet, sauté chopped onions for 5-7 minutes until soft.

Mix sautéed onions into the mashed potatoes with black pepper and paprika. Fold in cheddar cheese until melted.

Preheat oven to 200°C (400°F) and line a baking sheet with parchment.

Roll chilled dough into circles, place filling on one half, fold, and seal edges.

Brush tops with beaten egg and bake for 20-25 minutes or until golden brown.

Let cool on a rack for a few minutes before serving.