Chinese Pepper Steak with Onions

Ingredients:
1 pound flank steak or sirloin beef, thinly sliced against the grain
1 tablespoon soy sauce
1 tablespoon rice vinegar
1 tablespoon oyster sauce
1 tablespoon hoisin sauce
1 teaspoon sugar
2 tablespoons vegetable oil (for frying)
1 onion, sliced
2 bell peppers (1 green, 1 red), sliced
2 garlic cloves, minced
1-inch piece of ginger, minced
1 tablespoon cornstarch (for thickening)
½ cup water
Salt and pepper, to taste
Cooked rice or noodles, for serving
Instructions: