Step 1: To prepare the sauce, place the soy sauce with the vinegar, sugar, and cornstarch in a medium bowl. Whisk to combine and set aside.
Step 2: Heat a tbsp oil in a large skillet over high heat. Once the oil is hot, add the flank steak into the skillet. Season with salt and cool for about 8 minutes or until both sides are seared. Once cooked through, transfer the flank steak onto a plate.
Step 3: In the same skillet, add the rest of the 1 tbsp oil followed by the green bell peppers. Cook for about 4 minutes until tender. Add in the garlic and ginger. Stir and cook until aromatic.
Step 4: Add the beef back to the skillet. Pour the prepared sauce over and cook for about 12 minutes, stirring until the sauce is glossy.
Step 5: When done, remove from the heat. Enjoy with rice.