Begin by marinating the beef. In a medium bowl, combine the thinly sliced beef, soy sauce, and minced garlic and ginger. Allow it to marinate for at least 30 minutes for enhanced flavor.
In a separate bowl, mix the cornstarch, flour, salt, and black pepper. This will be the coating for the beef.
Heat the vegetable oil in a large wok or skillet over medium-high heat until shimmering.
While the oil is heating, remove the beef from the marinade and coat each piece in the cornstarch mixture, ensuring they are well-covered.
Once the oil is hot, carefully add the coated beef strips in batches, frying them until golden brown and crispy, approximately 3-4 minutes per batch. Use a slotted spoon to remove them and set them aside on paper towels to drain excess oil.
In the same wok, pour out most of the oil, leaving about 1 tablespoon. Add the sliced onion and red bell pepper, stir-frying for 2-3 minutes until they are slightly softened.
In a small bowl, whisk together the rice vinegar, sweet chili sauce, ketchup, brown sugar, chili flakes, and sesame oil. Pour this mixture over the vegetables in the wok.
Add the crispy beef back into the wok and toss everything together, ensuring the beef is well-coated in the sauce and mixed with the vegetables. Cook for an additional 1-2 minutes until heated through.
Remove from heat, garnish with chopped green onions, and serve immediately over rice or noodles.