Thinly slice the beef into bite-sized strips.

Slice the red and green bell peppers and onion, and mince the garlic.

Make the Sweet and Sour Sauce

In a small bowl, whisk together the soy sauce, ketchup, rice vinegar (or apple cider vinegar), brown sugar, salt, and black pepper. Set aside.

Heat the vegetable oil in a large skillet or wok over medium-high heat.

Add the sliced beef and stir-fry for 3-4 minutes until browned and cooked through.

In the same skillet, add the minced garlic, sliced onion, and bell peppers.

Stir-fry for 2-3 minutes until the vegetables are slightly softened but still crisp.

Return the cooked beef to the skillet with the vegetables.

Pour in the prepared sweet and sour sauce, stirring to coat everything evenly.

Add the pineapple chunks, if using, and cook for an additional 2-3 minutes until everything is well combined and heated through.

Serve the Sweet and Sour Beef hot over a bed of steamed rice or noodles, and enjoy!