In a bowl, combine the thinly sliced beef with soy sauce, minced garlic, brown sugar, salt, and pepper.

Mix well to ensure the beef is evenly coated with the marinade.

Cover the bowl and let the beef marinate in the refrigerator for at least 30 minutes.

In a separate bowl, mix together tomato ketchup and a splash of soy sauce to create the sauce base.

Adjust the sweetness and saltiness according to your taste preference.

Heat vegetable oil in a skillet over medium-high heat.

Add the marinated beef slices and cook until browned and cooked through, about 3-4 minutes per side.

Once cooked, transfer the beef to a plate and set aside.

In the same skillet, add sliced onions and diced tomatoes.

Simmer the veggies until they become tender and start to turn golden brown.

Add the cooked beef back to the skillet, along with pineapple chunks and the prepared sauce.

Stir well to combine and let it simmer for a few minutes until the flavors meld together.

Before serving, sprinkle some chopped green onions on top.