Step 1: Prepare the Beef

1. Slice the Beef: Slice the beef thinly against the grain to ensure a tender bite. Aim for strips that are about 1/4-inch thick.
2. Coat with Cornstarch and Flour: In a mixing bowl, combine cornstarch and flour, and toss the beef strips in this mixture until they’re fully coated. This coating helps achieve a crispy finish when fried.

Step 2: Make the Sauce

1. Combine Sauce Ingredients: In a bowl, mix the ketchup, sugar, soy sauce, vinegar, and chili paste. Stir until the sugar dissolves completely.
2. Adjust for Taste: Give the sauce a quick taste and adjust the sugar, vinegar, or chili paste based on your preference for sweetness, tanginess, or spice.

Step 3: Fry the Beef

1. Heat Oil in a Pan: Pour oil into a deep pan or wok, enough for shallow frying, and heat it over medium-high heat.
2. Fry the Beef in Batches: Add the beef to the pan in small batches, making sure not to overcrowd the pan. Fry each batch until golden brown and crispy, about 2–3 minutes per side. Remove and drain on paper towels.

Step 4: Stir-Fry Vegetables

1. Sauté Bell Peppers and Onion: In a separate pan or wok, add a small amount of oil and stir-fry the bell peppers and onions. Cook them until slightly tender but still crisp, about 2–3 minutes.
2. Combine Ingredients: Add the fried beef to the pan with the vegetables and stir gently to combine.

Step 5: Coat with Sauce and Serve

1. Add Sauce to the Pan: Pour the prepared sauce over the beef and vegetables, stirring everything until coated evenly.
2. Serve Hot: Serve immediately, garnishing with green onions or sesame seeds if desired.