Preparing the Beef: Begin by slicing the beef sirloin into thin strips. Ensuring the beef is thinly sliced for quick and even cooking is important. Tender cuts of beef, such as sirloin, work best for this recipe.

Marinating the Beef: Combine the sliced beef, minced garlic, soy sauce, calamansi juice, ground black pepper, and salt in a bowl. Mix well, ensuring that each piece of beef is evenly coated with the marinade. Let it marinate for at least 30 minutes to allow the flavors to infuse.

Cooking the Vegetables: Heat a large frying pan or wok over medium heat and add the cooking oil. Once the oil is hot, add the sliced onions and cook until they turn translucent and slightly caramelized. Vegetables will release their natural sweetness, adding depth to the dish.

Next, add the sliced bell peppers to the pan and stir-fry them for a few minutes until they become tender yet retain some crunch. The vibrant colors of the bell peppers will add visual appeal to the dish.

Combining the Beef and Vegetables: Increase the heat to high and add the marinated beef to the pan. Stir-fry the beef with the vegetables for approximately 5 minutes or until the beef is cooked to your desired level of doneness. Be mindful not to overcook the beef, as it may become tough.

Once the beef is cooked, taste the dish and adjust the seasoning if necessary. You can add more soy sauce, calamansi juice, or salt, according to your preference. Please give it a final stir to ensure all the flavors are well incorporated.