Spicy Sesame Beef Recipe

Name: Spicy Sesame Beef Recipe
Ingredients: 3 tablespoons oyster sauce, divided; 3 tablespoons soy sauce, divided; 5 tablespoons cornstarch (or cornflour), divided; 3 teaspoons toasted sesame oil, divided; ½ teaspoon garlic powder; A few pinches chili flakes, divided; 1 pound sliced steak; 1 teaspoon rice wine vinegar; 3 tablespoons sesame seeds, plus more for serving; 3 tablespoons neutral cooking oil, divided; 2 cloves garlic, finely chopped; 2 tablespoons brown sugar; 1 scallion, sliced, to serve