In a bowl, toss the sliced flank steak with cornstarch until well coated. Let it sit for about 10 minutes to absorb the cornstarch.

Heat vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the beef in batches, frying until crispy and browned, about 3-4 minutes per batch. Remove and drain on paper towels.

In the same skillet, add the sliced bell pepper and onion. Stir-fry for about 3-4 minutes until they are tender-crisp.

Add the minced garlic and ginger to the skillet, cooking for an additional 1 minute until fragrant.

In a small bowl, mix together soy sauce, rice vinegar, brown sugar, and chili paste. Pour this sauce over the vegetables in the skillet and stir to combine.

Return the crispy beef to the skillet, tossing everything together until the beef is well coated in the sauce, about 2 minutes.

Serve the crispy chilli beef over cooked white rice, garnished with chopped green onions and sesame seeds.