Marinate the Steak: In a bowl, whisk together soy sauce, minced garlic, freshly grated ginger, and brown sugar until well combined. Add thinly sliced steak to the marinade, ensuring each piece is coated evenly. Allow the steak to marinate for at least 15 minutes at room temperature or refrigerate for up to 2 hours.

Sear the Steak: Heat a large skillet or wok over medium-high heat and drizzle in a couple of tablespoons of oil. Add the marinated steak in batches, sear for 1-2 minutes on each side until browned but still tender. Remove and set aside.

Cook the Vegetables: In the same skillet, add a touch more oil if necessary, and toss in sliced bell peppers and onions. Stir-fry for 3-5 minutes until tender-crisp.

Thicken the Sauce: Mix cornstarch with a few tablespoons of water to make a slurry. Pour leftover marinade into the skillet with vegetables, then add the cornstarch slurry. Stir and simmer for about 1-2 minutes until the sauce thickens.

Combine & Serve: Return the seared steak to the skillet, mix thoroughly, and heat through for an additional minute. Serve over steamed rice or noodles.