Ingredients

• 1 tablespoon vegetable oil
• 1 to 1 1/2 pounds flank steak, sliced into 1 1/2-inch wide strips
• Kosher salt and freshly ground black pepper
• 1 large red onion, quartered and thinly sliced
• 1 large tomato, coarsely chopped
• 1 garlic clove, finely minced
• 1 teaspoon soy sauce
• 1 tablespoon red wine vinegar

Instructions

1. Heat the oil in a large wok or skillet over medium-high heat. Add the steak, season with salt and pepper, and stir-fry until the meat is browned on all sides, about 8 to 10 minutes.
2. Using tongs, transfer the steak to a plate and set aside. To the drippings in the pan, add the onion, tomato, and garlic; season with salt, and pepper. Cook and stir it until the onions are soft and the tomatoes start to break down, about 2 to 4 minutes. Return the beef to the pan, add the soy sauce and red wine vinegar. Cook for 1 minute, season with salt and pepper, to taste, and serve.