Slice beef thinly against the grain and toss in a bowl with cornstarch, a pinch of salt, and pepper. Set aside.

In a small bowl, whisk together soy sauce, brown sugar, sesame oil, sriracha or chili garlic sauce, and a splash of water. Set aside.

Heat vegetable oil in a large skillet or wok over medium-high heat. Add green beans and cook for 3–4 minutes until tender-crisp. Remove and set aside.

In the same skillet, add more oil if needed, then add beef in a single layer. Sear until browned, about 1–2 minutes per side. Work in batches if necessary.

Add minced garlic and ginger to the beef and stir-fry for 30 seconds until fragrant.

Return green beans to the skillet, pour in the sauce, and toss everything together. Simmer for 1–2 minutes until sauce thickens and coats the beef and beans.

Serve hot over a bed of steamed rice and garnish with green onions or sesame seeds if desired.