Mix flour, salt, butter, and lard in a food mixer until the mixture resembles breadcrumbs.
Swap to a dough hook and gradually add 220-260ml of cold water to form a dough.
Knead the dough for 4-5 minutes until smooth, then wrap and refrigerate for one hour.
Mix the potatoes, swede, onion, pepper, salt, parsley, and haddock together in a bowl.
Preheat the oven to 180°C and line a baking tray with parchment.
Divide the dough into four balls and roll each into a 25cm circle on a floured surface.
Spoon the filling onto half of each circle and top with a knob of butter.
Seal the edges with beaten egg and crimp them together to secure the filling.
Place pasties on the tray, brush with egg, and sprinkle with sea salt.
Bake for 40 minutes until golden-brown and cool for 10 minutes before serving.