1. Dice pepper steak into tiny 1/2 x 1/2 inch (one by one centimeter) pieces. Place into a large bowl and sprinkle with baking soda from high up. (This way you get a thin layer over all of the meat.) Next, add 1/4 cup water and massage the beef so the baking soda and water is uniformly distributed. The baking soda tenderizes the meat and prevents it from becoming tough and chewy when stir-frying.
2. Allow the meat to rest for 45–60 minutes. Rinse well in cold water and pat dry. Sprinkle lightly with salt and set aside.
3. While the meat is velveting, bring six cups of water to a boil. Add the ramen noodles, setting aside the spice packets for the sauce. Boil for four to five minutes, stirring gently several times, to break up the noodles. Drain and set aside.
4. In a large wok or frying pan, sauté the onion in three tablespoons of oil until soft and golden. Add the mushrooms and continue to sauté over high heat for an additional eight to 10 minutes or until soft and juices have evaporated. Remove onions and mushrooms from the pan and add more oil for stir-frying. Heat until almost smoking.
5. Add the beef to the pan in several batches, stir-frying quickly over high heat for about three to four minutes per batch, or until cooked through. Between batches, drain any liquid that forms and add more oil as necessary, taking care to heat it up to almost smoking again before adding more meat.
6. When all of the meat is cooked, return it all to the pan, along with the mushroom-onion mixture and the ramen noodles. Pour the sauce over everything, stirring well to coat. Cook for several minutes, stirring gently until heated through.
7. Transfer to a serving dish and top with sliced scallions and toasted sesame seeds. Serve immediately.
8. Combine sauce ingredients and set it aside.