Slice the flank steak against the grain into thin strips, about ¼ inch thick.

Chop two bell peppers and one onion into bite-sized pieces.

In a medium bowl, combine sliced steak with half of the soy sauce, black pepper, and cornstarch. Let marinate for at least 15 minutes.

Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat. Stir-fry chopped onions and peppers for 3-4 minutes.

Add remaining oil to the skillet, then sear the marinated beef strips for about 2 minutes on each side.

Whisk together remaining soy sauce, water, minced garlic, sugar, and sesame oil. Pour over cooked steak, add vegetables back, and cook for 2 additional minutes.