Thinly slice the beef sirloin or flank steak against the grain. Marinate beef with soy sauce, minced garlic, and grated ginger for at least 15 minutes.

Heat 2 tablespoons of olive oil or sesame oil in a wok over medium-high heat until shimmering.

Add marinated beef in a single layer, sear for 2-3 minutes without stirring, then flip and cook for another 1-2 minutes until just cooked. Remove and keep warm.

In the same wok, add more oil if needed, then stir in sliced onions and carrots. Sauté for about 2 minutes until translucent.

Add bell peppers, broccoli, and snow peas, cooking for an additional 3-4 minutes until vibrant and tender-crisp.

Return beef to the wok, stirring to combine. Mix cornstarch with water, pour over stir fry, and cook on high for 1-2 minutes until thickened.

Adjust seasoning with red pepper flakes or more soy sauce as desired. Stir well before serving.

Serve hot over rice, if desired, and enjoy!