Slice the steak into 1cm thick slices across the grain.
Whisk together soy sauce, sugar, cornflour and ginger in a bowl until the sugar has dissolved and the mixture is smooth. Place the steak slices into the marinade and stir until well coated.
Heat 1 tablespoon of vegetable oil in a wok or large frypan over medium-high heat and place 1/3 of the steak strips into the hot oil. Cook and stir until the beef is well browned; about 3 minutes then remove the beef from the wok to a bowl. Repeat twice more with the remaining beef. Set the cooked beef aside.
Return all the cooked beef to the hot wok then stir in the onion. Toss the beef and onion together until the onion begins to soften; about 2 minutes.
Stir in the green capsicum then cook and stir the mixture until the capsicum has turned bright green and started to become tender; about 2 minutes.
Add the tomatoes then stir everything together and serve.