In a large bowl, combine the soy sauce, cornstarch, brown sugar, black pepper, and crushed red pepper flakes (if using). Add the sliced flank steak and toss to coat. Let it marinate for at least 15 minutes.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated steak in a single layer and cook for about 2-3 minutes until browned. Remove the steak from the skillet and set aside.

In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced onion and bell peppers, and stir-fry for about 4-5 minutes until they are tender-crisp.

Add the minced garlic and cook for an additional 30 seconds until fragrant.

Return the cooked steak to the skillet and pour in the beef broth. Stir everything together and cook for another 2-3 minutes until heated through and the sauce thickens slightly.

Serve the pepper steak over cooked rice and garnish with chopped green onions.