Cook enough frozen fries according to the package to make 2 cups of cooked fries. Keep warm while preparing the stir-fry.
Cut 450g of beef sirloin across the grain into ¼in / 5mm strips and season with salt.
Heat 2 tablespoons of neutral oil in a large skillet over medium-high heat. Sear the beef in uncrowded batches for 1-2 minutes on one side, then about 1 minute on the other. Transfer to a plate.
Add 1 red onion, quartered, and 2 Roma tomatoes, quartered. Cook for about 5 minutes, adding a little oil if needed.
Stir in 2 ají amarillo peppers, seeded and minced, 3 garlic cloves, minced, 1 teaspoon of ground cumin, 1 tablespoon of tomato paste, ⅓ cup of soy sauce, and ⅓ cup of beef stock. Simmer for 5-7 minutes, until slightly thickened.
Return the beef and its juices, then toss in the fries. Serve over cooked white rice with chopped cilantro.
