Begin by preparing all the vegetables. Slice the beef sirloin thinly against the grain. Cut the broccoli into florets, slice the bell pepper, and trim the snap peas. Slice the onion and mince the garlic.
In a small bowl, mix the soy sauce, oyster sauce, cornstarch, sesame oil, and water. This will be your sauce mixture. Set aside.
Heat the vegetable oil in a wok or large skillet over medium-high heat. When the oil is hot, add the beef slices in a single layer. Stir-fry for about 3-4 minutes, or until browned and cooked through. Remove the beef from the wok and set aside.
In the same wok, add the onion and minced garlic. Cook for about 1 minute until fragrant.
Add the broccoli, bell pepper, and snap peas to the wok. Stir-fry for another 3-4 minutes until the vegetables are tender-crisp.
Return the cooked beef to the wok and pour the sauce mixture over the stir fry. Stir well to combine and cook for an additional 1-2 minutes, allowing the sauce to thicken slightly. Season with salt and pepper to taste.
Serve the beef stir fry over cooked rice or noodles. Enjoy!