Cut the sirloin steak into thin strips.

Place the steak strips in a large plastic bowl with a cover.

In a separate bowl, mix soy sauce, minced garlic, and oil.

Pour the mixture over the steak strips and toss to coat evenly.

Cover the bowl and marinate in the refrigerator for at least one hour.

When ready to cook, heat a frying pan or wok over high heat.

Pour the marinated meat mixture into the hot pan or wok.

Brown the meat thoroughly on all sides.

Add the green pepper strips, red pepper strips, sliced onion, and diced celery (if using).

Cover the pan and cook over low heat for 5-10 minutes, or until the vegetables are tender-crisp.

In a small bowl, stir together cornstarch and water to create a slurry.

Pour the cornstarch slurry into the pepper steak and stir continuously until the sauce thickens.

Add the diced whole tomatoes.

Cook for an additional 5-10 minutes, or until the meat is tender and the tomatoes are heated through.

Serve hot over boiled white rice.