Sizzling Chinese Pepper Steak with Onions

Ingredients

For the Marinade:
1 pound beef sirloin or flank steak, thinly sliced against the grain
2 tablespoons soy sauce
1 tablespoon oyster sauce
1 teaspoon cornstarch
1 teaspoon sesame oil
For the Stir-Fry:
2 tablespoons vegetable oil
1 large onion, sliced
1 red bell pepper, sliced
1 green bell pepper, sliced
2 cloves garlic, minced
1 teaspoon fresh ginger, grated
For the Sauce:
1/4 cup soy sauce
2 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)