Mix flour, sugar, and salt in a bowl.
Dissolve yeast in warm milk, let it sit for 5 minutes.
Add yeast mixture, melted butter, and egg to the flour mixture. Knead until smooth.
Let the dough rise for 1 hour.
Cook pork in a pan until browned. Add soy sauce, oyster sauce, onions, scallions, carrots, peas, and water. Cook until vegetables are tender.
Add cornstarch mixture to the pan, cook until sauce thickens.
Preheat oven to 350°F (175°C).
Divide dough into 12 pieces, flatten each piece, and fill with pork mixture. Seal and shape into buns.
Place buns on a baking sheet, let them rise for 15 minutes.
Bake for 20 minutes or until golden brown.