Thinly slice the flank steak or sirloin into ¼ inch strips, then pat them dry with paper towels.

In a mixing bowl, toss the beef strips with cornflour, salt, and white pepper until well-coated.

In a deep frying pan, heat vegetable oil over medium-high heat until it reaches 350°F (175°C).

Fry the beef strips in small batches for 2-3 minutes until golden brown and crispy.

In a separate bowl, whisk together the sweet chilli sauce, soy sauce, rice vinegar, brown sugar, tomato ketchup, minced garlic, grated ginger, sesame oil, and chilli flakes.

In a clean wok, sauté the red bell peppers and spring onions in a bit of oil for about 2 minutes until tender.

Pour the prepared sauce mixture into the wok with the sautéed vegetables, and heat through.

Add the crispy beef, tossing to combine until coated in the sauce.

Garnish with chopped spring onions and sesame seeds, and serve immediately over rice or noodles.