Prepare all ingredients by chopping the vegetables and slicing the meat.
Heat vegetable oil in a large pan or wok over medium-high heat.
Add the minced garlic and sliced onion; stir-fry until fragrant.
Add the meat and cook until browned.
Toss in the mixed vegetables and stir-fry for about 3-4 minutes.
Add the bean sprouts and stir for another minute.
In a small bowl, mix soy sauce, oyster sauce, and cornstarch with a bit of water.
Pour the sauce over the stir-fried ingredients and mix well.
Season with salt and pepper to taste.
Serve hot over rice or noodles.