To prepare the beef, slice the beef into thin strips. In a bowl, add the beef and sprinkle in the salt, then mix well. Add the egg white, Shaoxing wine, white pepper, and baking soda, then mix until evenly coated. Let the beef marinate while preparing the other ingredients.
To make the sauce, in a small bowl, whisk together the sweet chili sauce, ketchup, soy sauce, rice vinegar, and sugar.
To coat the beef, coat the marinated beef evenly with the cornstarch.
To fry the beef, heat the cooking oil in a pot over high heat to 350°F (177°C). Fry the beef in batches for about 2 minutes, until lightly golden, then remove and drain.
To refry the beef, increase the oil temperature to 375°F (191°C). Return the beef to the oil and fry again for about 1 minute, until golden and crispy. Drain well.
To stir-fry and serve, in a wok over high heat, add the cooking oil. Stir-fry the onion and red bell pepper for about 20 seconds, until fragrant. Pour in the sauce and bring to a quick simmer. Add the fried beef and toss quickly to coat. Add the scallions and serve immediately.