Begin by preparing your ingredients for Sweet and Sour Beef. Slice the flank steak or sirloin against the grain into thin strips, roughly ¼ inch thick. Chop the bell peppers and onion into bite-sized pieces. Mince the garlic and set aside.

In a large bowl, toss the sliced beef strips with cornstarch and a sprinkle of salt and pepper. Ensure each piece is evenly coated and allow to rest for about 10 minutes.

In a large skillet or wok, pour in about 2 tablespoons of vegetable oil and heat over high heat until shimmering.

Carefully add the coated beef strips to the skillet in a single layer. Sear for about 2-3 minutes until browned but not fully cooked through. Remove and set aside.

In the same skillet, add the chopped bell peppers, onion, and minced garlic. Stir-fry for about 3-4 minutes until crisp-tender.

Return the browned beef to the skillet. Add pineapple chunks, soy sauce, ketchup, rice vinegar, and brown sugar. Stir together and cook for another 2-3 minutes until the beef is cooked through.

To thicken the sauce, mix 1 tablespoon of cornstarch with 2 tablespoons of water and pour into the skillet while stirring continuously for 1-2 minutes.

Remove from heat and serve over steamed jasmine rice or stir-fried noodles.