In a bowl, combine the sliced flank steak with soy sauce and cornstarch. Mix well and let marinate for about 10 minutes.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated flank steak to the hot skillet and stir-fry for about 2-3 minutes until browned. Remove the steak from the skillet and set aside.
In the same skillet, add sliced onions, green bell pepper, and red bell pepper. Stir-fry for approximately 3-4 minutes until they are tender but still crisp.
Add minced garlic and grated ginger to the skillet, stirring for another minute until fragrant.
Return the cooked steak back to the skillet. Add the oyster sauce and rice vinegar, stirring everything together. Cook for an additional 2-3 minutes, allowing the flavors to meld.
Season with salt and pepper to taste.
Serve hot over cooked rice.