Slice the beef into thin strips and marinate with soy sauce, cornstarch, and a little sesame oil for 15–20 minutes.
Heat oil in a wok or large pan over medium-high heat.
Add the beef and stir-fry until browned and nearly cooked through. Remove and set aside.
Add garlic, ginger, and dried red chilies to the same pan. Stir-fry briefly until fragrant.
Add chopped bell peppers and cook for 2–3 minutes until slightly tender but still crisp.
Mix soy sauce, hoisin sauce, vinegar, sugar, and a little water in a bowl.
Return the beef to the pan, pour in the sauce, and stir until everything is well coated and the sauce thickens.
Add roasted peanuts and sliced green onions. Toss well and serve hot with steamed rice or noodles.