Slice the beef into 1/2-inch thick strips across the grain for tenderness.

In a bowl, whisk together the soy sauce, white sugar, cornstarch, and ground ginger until smooth and the sugar is dissolved.

Coat the beef slices thoroughly in the marinade, ensuring even coverage.

Heat 1 tablespoon of vegetable oil in a wok or large skillet over medium-high heat. Cook the beef strips in batches, stirring frequently until browned, about 3 minutes. Transfer the beef to a bowl and set aside.

In the same wok, return the seared beef and add the red onion. Toss and cook for about 2 minutes until the onion begins to soften.

Stir in the green bell pepper and continue cooking for 2 minutes until it becomes vibrant and tender.

Add the tomato wedges to the wok and stir everything together to combine the flavors. Serve the dish immediately.