Sizzling Chinese Pepper Stea

IngredientsFor the Marinade:1 pound beef sirloin or flank steak, thinly sliced against the grain2 tablespoons soy sauce1 tablespoon oyster sauce1 teaspoon cornstarch1 teaspoon sesame oilFor the Stir-Fry:2 tablespoons vegetable oil1 large onion, sliced1 red bell pepper, sliced1 green bell pepper, sliced2 cloves garlic, minced1 teaspoon fresh ginger, gratedFor the Sauce:1/4 cup soy sauce2 tablespoons hoisin sauce (optional)1 tablespoon cornstarch mixed with 2 tablespoons water (slurry for thickening)1/2 teaspoon black pepper (or more, to taste)1/4 teaspoon red pepper flakes (optional, for spice)