Prepare the Beef:
Slice the beef thinly against the grain into strips. Season with salt, pepper, and 1 tablespoon of soy sauce. Optionally, you can toss the beef with cornstarch to help thicken the sauce later.

Prepare the Vegetables:
Peel and cut the Chinese yam into thin slices or matchsticks. Slice the bell pepper and onion. Mince the garlic and grate or chop the ginger.

Cook the Beef:
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Once the oil is hot, add the beef in a single layer. Stir-fry for 2-3 minutes until the beef is cooked through and lightly browned. Remove the beef from the skillet and set it aside.

Stir-Fry the Vegetables:
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced onion, bell pepper, and garlic, and stir-fry for 2-3 minutes until they start to soften. Add the grated ginger and stir for another 30 seconds until fragrant.

Add the Chinese Yam:
Add the Chinese yam slices to the skillet and stir-fry for 3-4 minutes. The yam should become tender but still slightly crisp.

Make the Sauce:
In a small bowl, mix together the remaining tablespoon of soy sauce, oyster sauce (if using), rice vinegar, sesame oil, and chili flakes (if using).

Combine Everything:
Return the cooked beef to the skillet with the vegetables. Pour the sauce over the stir-fry and toss everything together to coat evenly. Stir-fry for another 1-2 minutes until everything is well combined and heated through.

Serve:
Transfer the stir-fry to a serving dish. Garnish with sesame seeds and sliced green onions. Serve on its own or with steamed rice for a complete meal.