1 pound beef sirloin, sliced thinly
2 tablespoons vegetable oil
2 cloves garlic, minced
1 tablespoon ginger, grated
1 onion, sliced
2 cups mixed vegetables (such as bell peppers, broccoli, and carrots)
1/4 cup soy sauce
2 tablespoons oyster sauce
1 tablespoon cornstarch
1/4 cup water
Salt and pepper to taste
Green onions, chopped (for garnish)