- Cut the cold butter into cubes, then put them evenly on a large cutting board; pour flour, a pinch of salt on top, and start chopping the butter with flour using a knife into medium crumbs.
- Whisk cold egg and water in a cup, pour on top of the flour mixture until the flour is moistened, and quickly grab the dough into a ball without kneading. If the dough does not come together, you can add more water. Your task is to ensure that all the flour is evenly wet, and the pieces of butter do not melt.
- Wrap the dough in cling film, and refrigerate for at least 30 minutes.
- For the filling, separate the fish fillet from the skin, rinse, dry with a napkin, and cut into pieces. Peel and cut the potatoes quite finely; cut the onion into half rings.
- Divide the cooled dough into two parts, roll out the first half on a surface sprinkled with flour.
- Wrap the dough on a rolling pin, and place on a baking sheet lined with parchment.
- Put the fish, potatoes and onions, season with salt and pepper to taste.
- Roll out the second part of the dough, and cover the pie, pinching the edges of the dough.
- Make a hole in the middle with a knife crosswise, and grease with an egg.
- Bake the pies for 30-40 minutes at 356F (180C).