In a bowl, mix warm water, yeast, and a pinch of sugar. Let it sit for 10 minutes until frothy.
In a large bowl, combine flour, sugar, and salt. Add yeast mixture and vegetable oil. Knead until smooth. Let it rise for 1 hour.
For the filling, sauté garlic and onion until fragrant. Add pork and cook until browned. Add soy sauce, oyster sauce, sugar, and water. Simmer for 20 minutes.
Add slurry to the mixture and cook until thickened. Season with salt and pepper. Let it cool.
Punch down the dough and divide it into 12 pieces. Flatten each piece and add filling. Seal and shape into buns.
Place buns on parchment paper and let them rise for 30 minutes.
Steam buns for 15-20 minutes until cooked through.