Creating an amazing Sweet and Sour Beef is easy when you follow these straightforward steps:

1. Prepare the Beef: Thinly slice the beef sirloin against the grain for tenderness. Season with salt and pepper.
2. Make the Sauce: In a small bowl, mix soy sauce, rice vinegar, brown sugar, ketchup, and cornstarch. Stir until the sugar and cornstarch dissolve.
3. Heat the Pan: In a large skillet or wok, heat vegetable oil over medium-high heat.
4. Cook the Beef: Add the sliced beef to the pan, stirring frequently for about 5-7 minutes, or until browned and cooked through. Remove the beef from the pan and set aside.
5. Sauté Vegetables: In the same skillet, add bell peppers and onion. Sauté for 3-4 minutes until they are slightly tender. Add garlic and cook for an additional minute.
6. Combine Ingredients: Return the cooked beef to the skillet. Add pineapple chunks and pour the prepared sauce over everything. Stir well to combine.
7. Thicken the Sauce: Let the mixture cook for another 3-5 minutes until the sauce has thickened and everything is heated through.
8. Adjust Seasoning: Taste the dish and adjust seasoning with additional salt and pepper if necessary.
9. Serve Immediately: Remove from heat and serve hot.

These simple instructions will guide you through creating this delightful dish without a hitch!