Place beef in a large bowl and toss with cornstarch, salt, and pepper, until evenly coated.
Heat 2 tablespoons vegetable oil in a large skillet or wok until shimmery. Working in batches if needed, add beef in a single layer and sear until beef is golden brown. Repeat cooking with remaining beef, adding additional 1-2 tablespoons oil to pan as needed. Transfer beef to a bowl and set aside.
Heat remaining 2 tablespoons oil in same pan. Add pepper, onion, zucchini, snow peas, and chili to pan and cook 3-5 minutes until tender. Add garlic and cook 1 minute until fragrant. Remove from heat for a moment.
In a small bowl, whisk soy sauce, oyster sauce, rice vinegar, fish sauce, and brown sugar until smooth.
Return pan with vegetables to stovetop and set over medium-high heat. Add beef strips, sauce mixture, and basil leaves to pan and toss over medium-high heat 2-3 minutes until sauce is thickened and coating vegetables and beef.
Season beef with salt and pepper to taste if needed and serve with green onions and rice. Enjoy!
Notes
Storing InstructionsStore beef in refrigerator up to 6 days.