Cut the duck breast skin in a check pattern. Heat some oil over medium-high heat. Pan-sear the duck breast skin side down in a frying pan until golden brown.

When it's 80% cooked, remove it from the pan. Let cool and rest for 5 minutes.

As the duck breast cools, make the sweet and sour sauce. Mix together black vinegar, sugar, and soy sauce. Set aside.

Using the same pan, heat some oil over medium-high heat. Add in sliced onions and cook until they begin to become translucent.

Add in sliced duck breasts and seasoning sauce. Stir fry for 1-2 minutes, bring the heat down, and simmer until the sauce thickens.

In a lotus leaf bun, or bao bun, place a handful of fresh cilantro and 2-3 pieces of sweet-and-sour duck breasts, then garnish with chopped spring onions.