If you're not using pre-sliced beef, first freeze it for 30 minutes to make it easier to cut.

Cut the beef into thin (1/4-inch) strips, against the grain (across the long lines of muscles in the meat).

Once cut, add the beef to a bowl with the sesame oil, baking soda, cornstarch, and salt, and mix well, massaging it with your hands. Set aside for 15-30 minutes to "marinate".

Meanwhile, mix the sauce ingredients (minus the garlic and ginger) in a small bowl. Mix the cornstarch with water into a slurry in a separate bowl.

Mince the garlic and ginger. Cut the green onions so the green parts are cut lengthwise (into long strips) and the white parts are sliced into rounds. Slice the bell peppers (removing the ribs/seeds) and onions (pole to pole) into even-sized strips.

Add about 1/3 of the oil to a large skillet/wok over medium-high heat. Once hot, add the beef in a single layer (cook in batches if necessary) and cook for 2-3 minutes per side, untouched, until it's no longer pink.

Add another 1/3 of the oil to the pan and stir-fry the peppers and onions until tender-crisp (2-3 minutes).

Heat the remaining oil, reduce the heat to medium, and sauté the garlic and ginger for just 1-2 minutes.

Add the stir-fry sauce and cornstarch slurry (mix/whisk well beforehand) to the pan and cook for 1-2 minutes, stirring constantly, until it becomes glossy and thickens. If the sauce isn't thick enough, add more cornstarch slurry. If it's too thick, add water/beef broth.

Add the cooked ingredients back into the pan with the green onions, stir, and serve – enjoy optionally sprinkled with sesame seed.