Start by slicing the flank steak against the grain into thin strips, about 1/4 inch thick.
In a mixing bowl, combine the sliced beef with 3 tablespoons of cornstarch and 1/4 teaspoon of salt. Toss until the beef is evenly coated.
In a large frying pan or wok, heat 1 cup of canola oil over medium-high heat.
Once the oil is hot, carefully add the coated beef strips in small batches. Fry for about 2-3 minutes until golden brown and crispy. Remove and drain on paper towels.
In the same pan, add the minced garlic, grated ginger, sliced yellow onion, and red bell pepper. Stir-fry for about 3-4 minutes.
In a mixing bowl, whisk together the remaining 1/4 cup of cornstarch, 1/2 cup of water, brown sugar, ketchup, hoisin sauce, soy sauce, oyster sauce, sweet chili sauce, crushed red pepper flakes, and rice vinegar. Pour this sauce over the stir-fried vegetables in the pan.
Stir the sauce and vegetables together, allowing the sauce to thicken for about 2 minutes. Finally, add the crispy beef back to the pan.
Serve hot over rice or noodles for a complete meal.
