Start by slicing the flank steak against the grain into thin strips. In a mixing bowl, whisk the large egg and add the sliced beef. Mix well to combine.
In another bowl, combine the cornstarch and salt. Dredge the egg-coated beef strips in the cornstarch mixture until fully coated.
In a large frying pan or wok, heat the vegetable oil over medium-high heat.
Once the oil is hot, carefully add the coated beef strips in batches and fry for about 4-5 minutes until golden brown and crispy. Use a slotted spoon to remove the beef and place it on a paper towel-lined plate.
In the same pan, add the chopped red bell pepper and onion. Sauté for about 2-3 minutes until tender. Add the minced garlic and crushed red pepper, cooking for an additional minute.
In a small bowl, whisk together the ketchup, hoisin sauce, sweet chili sauce, and soy sauce. Pour this sauce over the sautéed vegetables and stir to combine. Allow it to simmer for a minute.
Add the crispy beef back into the pan, tossing everything together to ensure the beef is coated in the sauce. Cook for another minute to heat through.
Transfer the Beijing Beef to a serving platter and garnish with chopped scallions. Serve immediately and enjoy!