Combine cornstarch and water in a small bowl, whisking until completely smooth with no lumps. Add soy sauce, oyster sauce, sesame oil, rice vinegar, and beef broth to the cornstarch mixture. Stir well and set the sauce aside—this prevents clumping when it hits the hot pan later.

Pat beef sirloin slices dry with paper towels to ensure proper browning and crust development. Season with white pepper just before cooking to prevent moisture loss during prep. This crucial step guarantees that your beef achieves restaurant-quality searing.

Heat vegetable oil in a large wok or skillet over high heat for 2 minutes until it shimmers. Working in two batches to avoid crowding, sear the beef for 60-90 seconds per side until deeply browned. Transfer cooked beef to a clean plate and set aside—do not skip the resting period.

In the same wok, add garlic and ginger, stirring constantly for exactly 30 seconds until fragrant. Add sliced red bell pepper and snow peas, tossing continuously for 2-3 minutes until vegetables begin to soften slightly but remain crisp. This tender beef and snow peas combination requires precise timing to maintain that signature texture contrast.

Return the seared beef to the wok along with any accumulated juices. Pour the prepared sauce over everything and toss gently but thoroughly for 1-2 minutes until the sauce thickens and coats all ingredients evenly. The mixture should transform from thin liquid to glossy, clinging sauce within seconds.

Remove from heat and stir in chopped green onions. Taste and adjust seasoning with additional soy sauce or rice vinegar as needed. Serve immediately while the Asian beef stir fry remains hot and the vegetables maintain their ideal texture.