Prepare the Fries: Heat vegetable oil in a deep skillet or fryer. Fry the potato fries until golden and crispy. Drain on a paper towel and set aside.
Marinate the Beef: In a bowl, mix beef strips with soy sauce, vinegar, garlic, salt, and pepper. Let it marinate for 10-15 minutes.
Sauté the Beef: Heat 2 tablespoons of vegetable oil in a large skillet over high heat. Add the marinated beef and stir-fry for about 2-3 minutes until browned. Remove the beef from the skillet and set aside.
Cook the Vegetables: In the same skillet, add the sliced onions, tomatoes, and aji pepper. Stir-fry for 2 minutes until the onions become tender but still slightly crisp.
Combine Ingredients: Return the beef to the skillet with the vegetables. Add the chopped cilantro and stir to combine. Cook for another 2 minutes to meld flavors.