Heat 1 tablespoon of oil in a large pan over high heat. Add the steak, season with salt and pepper, and cook until browned, 5-6 minutes. Remove from the pan.
Heat the remaining tablespoon of oil in the same pan, then add the red onion and cook for about 5 minutes, until softened and browned. Add the tomato, garlic, and aji amarillo paste, and cook for another 5-7 minutes, until the tomatoes have released some of their juices, but are still intact.
Add the soy sauce and vinegar and stir to combine, let cook for 1 minute.
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Add the steak, fries, and cilantro. Toss gently to coat the fries in the sauce.