Snap green beans into 2–3-inch pieces.

Slice mushrooms and onion and set aside.

Cut the flank steak into thin strips by slicing with the grain into 2-inch-wide pieces, then slicing across the grain into ⅛-inch slices.

In a medium bowl, whisk together soy sauce, cornstarch, rice vinegar (or lime juice), sugar, and black pepper.

Add the beef slices, mix well, and let marinate for 10–15 minutes while preparing the vegetables.

Heat 1 tablespoon oil in a large wok or skillet over high heat.

Add the green beans and stir-fry for 1 minute.

Pour in ¼ cup water, reduce heat to medium, cover, and steam for 6–8 minutes, until tender-crisp.

Remove the beans and any remaining liquid; set aside.

Increase heat to high, then add 1 tablespoon oil to the wok.

Add ginger slices and sauté for 30 seconds until fragrant.

Add onion and mushrooms; stir-fry for 2–3 minutes until the onion softens and mushrooms begin to brown.

Remove ginger slices and discard.

Add the remaining 1 tablespoon oil to the pan.

Add the marinated beef and cook for 2–3 minutes, stirring constantly, until browned and no longer pink.

Return the green beans, mushrooms, and onions to the pan.

Toss everything together and stir-fry for 1–2 minutes until evenly coated and heated through.

Taste and season with salt or extra black pepper if needed.

Serve hot over a bed of steamed jasmine rice or noodles, garnished with a sprinkle of freshly cracked pepper or toasted sesame seeds for extra flavor.

For tender beef, slice against the grain as thinly as possible.

Add a drizzle of sesame oil or a sprinkle of red pepper flakes before serving for extra depth.

To make it vegetarian, replace the beef with tofu or tempeh — the sauce pairs beautifully with both!