How to Make It
Heat oil in a large skillet over medium-high. Sprinkle sirloin with black pepper and 1 teaspoon of the salt. Cook, stirring occasionally, until browned, about 2 minutes. Transfer sirloin to a plate.
Add onion, bell pepper strips, serrano chiles, and garlic; cook, stirring occasionally, until onions are translucent, 2 to 3 minutes. Add sirloin and any accumulated juices, tomatoes, soy sauce, vinegar, and remaining 1/2 teaspoon salt. Cook, stirring occasionally, 2 minutes. Remove from heat. Add French fries, and gently toss to combine. Sprinkle with scallions and cilantro.