Prepare the Sauce
In a medium bowl, whisk together the beef broth, soy sauce, rice vinegar, mirin, sesame oil, and cornstarch. Set aside.
Cook the Beef
Heat 2 tablespoons of vegetable oil in a large skillet over medium-high heat. Add the beef slices in a single layer, seasoning with a pinch of kosher salt and ground black pepper. Sear the beef until browned on both sides, working in batches to avoid overcrowding the pan. Remove the beef and set it aside.
Cook the Vegetables
In the same skillet, add 1 tablespoon of vegetable oil. Stir-fry the red and green bell peppers along with the onion for 2–3 minutes, or until they are just starting to brown on the edges. Remove the vegetables from the skillet and set aside.
Sauté Aromatics
Reduce the heat to medium-low. Add a small amount of oil if needed, and stir in the minced ginger and garlic. Cook for about 1 minute, stirring constantly, until fragrant.
Thicken the Sauce
Whisk the sauce again to ensure the cornstarch is fully incorporated. Pour the sauce into the skillet, stirring constantly, and cook until it thickens to your desired consistency.
Combine and Serve
Return the beef and vegetables to the skillet, stirring to coat everything evenly in the sauce. Let the dish warm for a few minutes before serving. Serve hot over steamed rice or noodles.