🥩 Prepare the Steak:
Slice the flank steak into thin strips against the grain. This helps to keep the steak tender and easy to chew.

🥄 Make the Sauce:
In a small bowl, mix together the soy sauce, oyster sauce, brown sugar, cornstarch, rice vinegar (if using), and beef broth (or water). Stir well until the sugar and cornstarch are dissolved. Set aside.

🔥 Stir-Fry the Steak:
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced steak to the pan in batches to avoid overcrowding. Stir-fry for 2-3 minutes until the steak is browned on all sides. Remove the steak from the pan and set it aside.

🧅 Cook the Vegetables:
In the same pan, add the onions, bell pepper, and mushrooms (if using). Stir-fry for 3-4 minutes until the vegetables begin to soften but still retain some crunch.
Add the minced garlic and ginger (if using) and stir-fry for another 30 seconds until fragrant.

🍲 Combine Steak and Sauce:
Return the cooked steak to the pan with the vegetables.
Pour the sauce mixture over the steak and vegetables, stirring to coat everything evenly.
Bring the sauce to a simmer and cook for another 3-4 minutes until the sauce has thickened and everything is heated through.

🍽️ Serve:
Serve the pepper steak over steamed white rice or noodles.

💡 Optional Add-ins:
You can add sliced bamboo shoots or water chestnuts to give the dish more depth.
If you want a spicier kick, add some crushed red pepper flakes or a chopped jalapeño to the sauce.