In a small bowl, sprinkle yeast over lukewarm water. Let sit for a minute, then stir to combine. Let sit until frothy, about 10 minutes.
In the bowl of a large food processor fitted with a dough blade or a large bowl, combine flour and sugar. Process or mix in the yeast with water and vegetable oil until dough comes together. On a lightly floured surface, knead until smooth and elastic. Divide the dough into 24 equal pieces. Cover and let rest while filling is prepared.
In a large wok or pan, drizzle vegetable oil over medium heat. Once heated, add onion, mushroom, and spinach. Cook until onions and mushrooms are soft and spinach is wilted. Remove to a large bowl and allow to cool.
Once the vegetables have cooled, add the pork, soy sauce, sesame oil, garlic, and salt. Mix until combined.
Cut out 4x4 inch squares of parchment paper.
Take a piece of the rested dough and flatten on work surface. Use a small rolling pin to roll into a 1/8 inch thick circle. Place a heaping tablespoon to 2 tablespoons of filling in the center. Fold the edges of the circle upward to cover the filling. Gently twist and pinch the top to seal. Repeat with remaining dough and filling. Place each bun on prepared parchment squares. Cover the buns with a towel and let rest for about 40 minutes.
Place buns with parchment in steamer basket and steam over high heat until puffy, firm, and center is cooked through, 20-25 minutes. Repeat with remaining buns.