In a large mixing bowl, combine flour, sugar, and instant yeast. Add warm milk and vegetable oil; mix until a shaggy dough forms.
Knead the dough on a floured surface for about 8-10 minutes until smooth and elastic. Place in a greased bowl, cover, and let rise for about an hour until doubled in size.
For the filling, heat sesame oil in a skillet over medium heat. Sauté minced garlic and grated ginger until fragrant.
Add chopped beef along with soy sauce, hoisin sauce, and rice vinegar. Simmer for about 10-15 minutes until beef is tender. Stir in sliced green onions before removing from heat.
Punch down the risen dough and divide into 12-14 equal portions. Roll each into discs and fill with the beef mixture. Pinch edges to seal.
Steam the filled buns in a steamer basket for 12-15 minutes until fluffy.