In a bowl, combine the soy sauce, cornstarch, brown sugar, black pepper, and a pinch of salt. Mix well to create a marinade for the steak.
Add the sliced flank steak to the marinade and let it sit for about 5 minutes to absorb the flavors.
In a large skillet or wok, heat the vegetable oil over medium-high heat.
Add the marinated steak to the hot skillet. Stir-fry for about 3-4 minutes, or until the steak is browned and cooked through. Remove the steak from the skillet and set it aside.
In the same skillet, add the sliced onion, red bell pepper, green bell pepper, and minced garlic. Stir-fry the vegetables for about 4-5 minutes until they are tender-crisp.
Return the cooked steak to the skillet with the vegetables and mix well. Cook for an additional 2 minutes to allow the flavors to meld together.
Adjust seasoning if needed, adding more salt or pepper to taste.
Serve the pepper steak hot over cooked rice if desired.