Begin by slicing the flank steak against the grain into thin strips. In a bowl, combine the sliced steak with Chinese black vinegar, soy sauce, Chinese five spice, and cornstarch. Mix well and let it marinate for at least 15 minutes.
In a large skillet or wok, heat 2 tablespoons of vegetable oil over medium-high heat until it shimmers.
Add the marinated beef to the skillet in a single layer. Sear for 2-3 minutes on each side until golden-brown. Remove from skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Toss in the yellow onion, sugar snap peas, broccoli, red bell pepper, and garlic. Stir-fry for 4-5 minutes until vibrant and slightly tender.
Return the cooked beef to the skillet, pour in the Szechuan sauce, and mix everything thoroughly. Cook for an additional 2-3 minutes.
Remove from heat and garnish with chopped green onions. Serve hot over steamed rice or noodles.
