Place the beef tenderloin on a large cutting board and slice into ¼ inch thick strips, season with salt and pepper to taste and place in a large bowl.
In a small bowl whisk together the soy sauce, vinegar, sriracha, and cumin. Pour half of this mixture over the beef and stir to combine.
While the beef is marinating, slice the red onions and tomatoes into chunks about ¼-1/2 inch thick, and mince the garlic.
Heat the oil in a large skillet or wok over high heat.
Add half of the beef strips to the pan and sear the meat until browned on one side, then flip and sear for 1-2 more minutes. Remove the meat to a clean plate and repeat with the remaining beef.
Add the onions to the pan and cook, stirring constantly for 3 minutes, then add the tomatoes and garlic to the pan and cook, stirring constantly for one minute.
Return the beef to the pan and pour in the cooked french fries, then the remaining marinade. Toss to combine. Serve immediately with rice on the side if desired.