Slice the beef thinly against the grain. In a bowl, mix the beef slices with 1 tbsp soy sauce, 1 tbsp cornstarch, and a pinch of salt. Let it marinate for 10 minutes.

While the beef is marinating, slice the onions, bell peppers, and tomatoes into thin strips.

Heat 1 tbsp of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry until browned and cooked through, about 3-4 minutes. Remove the beef from the skillet and set aside.

In the same skillet, add the remaining 1 tbsp of oil. Add the sliced onions, bell peppers, and a pinch of salt. Stir-fry for about 5 minutes or until the vegetables are tender-crisp.

Add the minced garlic and grated ginger to the skillet, and stir-fry for 30 seconds until fragrant.

Add the tomatoes and continue to stir-fry for another 2 minutes. Return the beef to the skillet.

In a small bowl, mix the remaining 2 tbsp of soy sauce with 3 tbsp of water. Pour this sauce over the beef and vegetables in the skillet.

Stir everything together, making sure the beef and vegetables are coated with the sauce. Cook for another 1-2 minutes until the sauce has slightly thickened.

Taste and adjust the seasoning with salt and black pepper if needed. Serve hot over steamed rice.