In a small bowl, mix soy sauce, water, cornstarch, ground black pepper, and sugar if using; set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced flank steak and stir-fry until browned but not fully cooked, about 3-4 minutes; remove and set aside.
In the same skillet, add sesame oil if using, then add minced garlic and sliced onions; stir-fry for 2 minutes.
Add bell peppers and continue to stir-fry until they start to soften, about 3-4 minutes.
Return the steak to the skillet and pour the prepared sauce over the ingredients.
Cook while stirring until the sauce thickens and the steak is cooked through, about 3-5 minutes.
Serve hot with steamed rice or noodles if desired.