To make the filling: In a wok or frying pan, heat vegetable oil and cook onions until tender and translucent, about 5 minutes. Add roast pork and stir fry for 3 minutes. Add oyster sauce, sugar, sesame oil, dark soy sauce, cornstarch, and chicken stock. Let it simmer on low heat, until the sauce is thickened. Remove from heat and let it cool in the refrigerator for at least one hour.

In a mixing bowl, mix together all of the ingredients for the outer dough until well combined. Cover the dough with plastic wrap and let it rest in the refrigerator for 15 minutes. Divide evenly into 12 pieces, about 14 grams each.

In another mixing bowl, mix together all of the ingredients for the inner dough until well combined. Wrap the dough with plastic wrap and let it rest in the refrigerator for 15 minutes. Divide evenly into 12 pieces, about 13 grams each.

Preheat the oven to 400°F. Line a baking sheet with parchment paper.

Flatten each piece of outer dough, place an inner dough ball in the center, and cover it completely. Using a rolling pin, flatten the dough into an oval. Roll it up by hand, tightly, turn 90 degrees, and flatten it again with a rolling pin. Roll it up by hand for the second time, turn 90 degrees, and flatten it with a rolling pin into a rectangular shape.

Place a heaping tablespoon of the pork filling in the center. Fold both sides toward the center. Lay the pastry seam side down on the prepared baking sheet, spacing about two inches apart. Use a fork to seal the edges of the pastries. Repeat with the rest of the dough. Brush the top of the pastry with egg wash and sprinkle with sesame seeds. Bake for 20-25 minutes, or until they turn golden brown. Serve warm.