In a large bowl, combine flour and salt. Add cold cubed butter and rub into the flour until the mixture resembles breadcrumbs.
Add cold water and gently mix until a dough forms. Do not knead. Divide into two portions, wrap, and refrigerate for at least 30 minutes.
Heat oil in a skillet over medium heat. Add onions and sauté until soft.
Add minced beef and cook until browned. Stir in seasoning cube, salt, garlic powder, cayenne pepper, diced carrots, and potatoes.
Pour in most of the water, reserving a small amount. Cover and simmer for 10–12 minutes until vegetables are tender.
Mix cornstarch with the reserved water to form a slurry. Stir into the filling and cook until thickened. Allow the filling to cool completely.
Preheat oven to 350°F (175°C).
Roll out chilled dough on a lightly floured surface to about 1/8-inch thick. Cut into circles using a round cutter or bowl.
Brush edges with egg wash. Add 1–2 tablespoons of filling to the center of each circle. Fold over and seal edges with a fork.
Transfer pies to a baking tray. Brush tops with egg wash and poke small holes with a fork for ventilation.
Bake for 30–40 minutes until golden brown. Cool slightly before serving.