In a stand mixer, combine the flour, salt, soy milk, yeast, baking powder, and sugar. Using a dough hook, or your hands, knead for 5-10 minutes until the dough comes together smoothly. Add a little more soy milk if your dough is too coarse.
Cover and let rise until doubled, or about an hour.
Quick pickle some red onion by adding some thin sliced red onion to a jar or bowl. Cover halfway with apple cider vinegar or white vinegar and a little sugar. Cover the rest in boiling water and let sit for at least 30 minutes. Set aside. Alternatively, make these ahead of time and place them covered in the fridge for up to 3-5 days.
Uncover the dough, punch down, and knead for a couple of minutes.
Divide the dough into ten equal pieces. Use a bench knife and scale to make this an easy process. Roll each piece into a tight ball and cover them with plastic wrap on a plate to prevent them from drying out. Cut ten squares of parchment paper to place each bun on to prevent sticking.
One at a time, using a small rolling pin roll each ball of dough into a circle that is roughly twice as long as it is wide (about 2-2.5 by 4-5 inches). Brush the top with a light coat of oil and fold in half. Place it on one piece of parchment paper. Place the buns in a steamer basket and cover. Let them rise for another 15-30 minutes to let them rise a second time. I used two steamer baskets to fit all ten buns. While the buns rise, heat a large pot of water to boil. Once the buns have risen, place the steamer baskets over the boiling water and steam for 10 minutes over medium heat.
While the buns are steaming, toss the steak strips in a bowl with the sesame oil, soy sauce, rice wine vinegar, brown sugar, sesame seeds, ginger, garlic, pepper flakes, and sesame seeds. Once evenly coated, saute over medium heat for 5-7 minutes until the steak strips are heated through and the sauce is bubbly and thick. Set aside.
Remove the steam baskets from the pot, but do not remove the lid. Let them sit and rest for 5 minutes. Cool on a wire rack.
Assemble the bao buns with a few steak strips, some pickled onion, carrot, cucumber, and green onions. Top with additional sesame seeds, chopped peanuts, pepper flakes, or any desired sauces.
Serve warm. Store any leftovers in an airtight container in the fridge for a couple of days. Buns can be frozen in an airtight container for up to 3 months. Thaw in the fridge or at room temp.