In a medium bowl, combine the sliced flank steak with 2 tablespoons of soy sauce and 1 tablespoon of cornstarch. Toss to coat evenly and let marinate for at least 10 minutes.
In a small bowl, whisk together the soy sauce, hoisin sauce, rice vinegar, chili garlic sauce, sesame oil, and sugar. Set aside.
In a large skillet or wok, heat 1 tablespoon of vegetable oil over high heat. Add the marinated beef and cook until browned and cooked through, about 3-4 minutes. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining 1 tablespoon of vegetable oil. Add the minced garlic and ginger and stir-fry until fragrant, about 30 seconds. Add the sliced red and green bell peppers and onion. Stir-fry for 3-4 minutes, until the vegetables are tender-crisp.
Return the cooked beef to the skillet. Pour the sauce over the beef and vegetables, stirring to coat everything evenly. Cook for an additional 1-2 minutes, until the sauce is heated through and slightly thickened.
If using, stir in the roasted peanuts. Garnish with sliced green onions and sesame seeds, if desired.
Serve immediately over cooked rice.