Sizzling Chinese Pepper Steak with Onions

Ingredients

1 lb flank steak, thinly sliced against the grain
2 tablespoons soy sauce
1 tablespoon cornstarch
1 tablespoon vegetable oil
1 large onion, thinly sliced
2 bell peppers (any color), thinly sliced
3 cloves garlic, minced
1 tablespoon ginger, minced
1/4 cup soy sauce
1/4 cup beef broth
2 tablespoons oyster sauce
1 tablespoon hoisin sauce
1 teaspoon sugar
1/2 teaspoon black pepper
1/4 teaspoon red pepper flakes (optional)
Cooked rice (for serving)
Green onions, chopped (for garnish)