Cut the flank steak against the grain into 1/4-inch slices and place in a bowl with the soy sauce and baking soda. Stir to combine. Let the meat marinate for 30 minutes and then drain the beef of excess marinade.
While the meat is marinating, combine all the sauce ingredients in a small bowl. Mix well and set aside.
Pour the vegetable oil into a heavy-bottomed pot until it reaches a depth of 3 inches. Heat the oil to 350°F.
Dredge the beef on both sides with cornstarch. Shake off the excess.
Fry the beef in the vegetable oil in small batches for about 3 to 4 minutes or until golden brown on both sides. Drain the beef on a wire rack.
Heat a wok on high with a few tablespoons of the vegetable oil you used to fry the beef. Sauté the onions and red bell peppers until softened. Add the garlic and ginger and sauté until fragrant. Add the sauce to the wok and stir a few times. Then add the fried beef. Stir to coat the beef and vegetables with the sauce.