When you’re ready to cook, drain the extra liquid from the bowl of beef. Add the cornstarch. Stir to coat the beef, until it forms an uneven coating with a little dry cornstarch left unattached.
Heat oil in a large nonstick skillet (or a cast iron pan) over medium high heat until hot. Add the beef and spread into a single layer in the skillet. Separate the beef pieces with a pair of tongs or chopsticks.
Cook without touching the beef for 2 to 3 minutes, or until the bottom turns golden. Flip to brown the other side, 1 to 2 minutes. Transfer the beef to a big plate and remove the pan from the stove. Let cool for 2 to 3 minutes.
Place the pan back onto the stove and turn to medium heat. You should still have 1 to 2 tablespoons of oil in the pan. If not, add more.
Add the onion, pepper, and dried chili pepper. Cook and stir for 1 minute.
Add the ginger and garlic. Stir a few times to release the fragrance.
Stir the sauce again to dissolve the cornstarch completely. Pour into the pan. Stir and cook until it thickens a bit.
Add back the beef pieces. Stir to coat beef with sauce.
Transfer everything to a plate immediately and garnish with roasted sesame seeds, if using.
Serve hot over steamed rice as a main dish.