In a mixing bowl, combine thinly sliced flank steak with soy sauce, ground black pepper, and cornstarch. Toss until the beef is evenly coated, then let it marinate for 15-30 minutes at room temperature.

While the beef marinates, whisk together the stir fry sauce ingredients until smooth and set aside.

Heat a large skillet or wok over medium-high heat, adding vegetable oil. Cook the marinated beef in batches for 2-3 minutes until lightly golden and cooked through.

In the same skillet, add more oil if necessary, and stir-fry onions, bell peppers, and carrots for 3-4 minutes. Add broccoli and stir-fry for an additional 2 minutes.

Return the cooked beef to the skillet, pour in the prepared sauce, and cook for about 2 minutes, stirring continuously until the sauce thickens.

Remove from heat, stir in chopped scallions and garnish with sesame seeds if desired. Serve over steamed rice or cauliflower rice.