To make the filling, heat 1 tbsp of the oil in a skillet over medium heat. Add the garlic, ginger, and green onions and fry for 1 minute.
Add the red bell peppers, shiitake mushroom and ground beef and fry until the beef no longer pink.
Add in the soy sauce, mirin, oyster sauce and Chinese black bean paste. Remove from heat and cool for 10-15 minutes.
While the filling is cooling, whisk together the dry ingredients (flour, baking powder and salt) in a large mixing bowl until thoroughly combined.
Mix in the Greek yogurt and stir until a shaggy dough forms. If the flour mixture seems too wet add a little more flour. If it looks too dry, add a few drops of cold water.
Turn out the dough onto a clean floured work surface and knead until smooth, about 3-4 minutes.
Roll the dough into a long sausage shape and then divide into 8 equal portions.
Keeping the remaining dough portions covered with a clean dishcloth (to prevent drying) use a rolling pin to roll a piece of dough into a thin round, about 3 inches in diameter and 1/4 inch thick. For best results, keep the dough on the thin side as it will expand during the steaming process.
Add about 2 tablespoons of filling to the center of the round and then gather the edges in the center to form a small bundle. Alternatively you can pleat the buns to make a spiral shape or even fold in half to make a half-moon shape.
Heat the vegetable oil in a large skillet over medium high heat. Place the buns smooth side down in the pan.
Add the water and cover with a tight fitting lid. Steam the buns for 8 minutes until the water has evaporated.
Remove the lid and continue frying the buns for about 2 minutes until the bottom is golden brown and crunchy.
To serve, top with chili oil and chopped cilantro or Chinese black vinegar.