Nigerian Meat Pies

Name: Nigerian Meat Pies
Ingredients: For the Beef Filling:; 1 tablespoon (8g) cornstarch; 3 tablespoons (45ml) vegetable or other neutral cooking oil, divided; 8.8 ounces (250g) ground beef, at least 20% fat; Kosher salt; 1/2 small yellow onion (1.7 ounces; 50g), finely diced; 2 small Yukon gold or yellow potatoes (4.2 ounces; 120g total), peeled and cut into 1/2-inch dice; 1 small carrot (2.6 ounces; 75g), peeled and cut into 1/4-inch dice; 1 teaspoon Nigerian-style curry powder (see note); 1 teaspoon dried thyme; 1 cup (235ml) Nigerian-style stock or water; 1/2 small green bell pepper (1.7 ounces; 50g), stemmed, seeded, and finely chopped, divided; Freshly ground black pepper; For the Dough:; 5.6 ounces all-purpose flour (160g; 1 1/4 cups); 1 ounce whole wheat flour (30g; 1/4 cup); 1 teaspoon (3g) Diamond Crystal kosher salt; for table salt, use half as much by volume or the same weight; 1/2 teaspoon baking powder; 3.1 ounces unsalted butter (90g; 6 tablespoons), cold; 1 large egg (1 1/4 ounces; 50g), lightly beaten; 1/4 cup (60ml) ice-cold water, plus more as needed; To Assemble and Bake:; 1 large egg yolk (15g) lightly beaten with 1 tablespoon (15ml) milk (any fat percentage)