Toss the beef with half of the salt, the black pepper, 1 tablespoon soy sauce, and the vinegar. Let it stand for 10 minutes.
Heat a large skillet or wok over high heat. Add 1 tablespoon oil and sear the beef in a single layer, working in batches if necessary, until browned. Transfer to a bowl.
Stir-fry the vegetables:
Add the remaining oil to the hot skillet, then add the onion and cook for 1 minute.
Stir in the garlic, cumin, and aji amarillo paste and cook for 30 seconds.
Add the tomatoes and cook for 1 to 2 minutes, just until they begin to soften but still hold their shape.
Finish and serve:
Return the beef to the skillet. Add the remaining soy sauce and beef stock, then toss for 1 to 2 minutes until glossy.
Fold in the cilantro and remove from the heat.
Serve immediately with hot French fries and white rice.
Notes
Slice the beef across the grain for the most tender texture.
Keep the tomatoes in large wedges so they do not disappear into the sauce.
Reheat leftovers in a hot skillet and add fresh fries just before serving.