Place the steaks on a plate and season generously with salt and pepper. Press the rosemary into each side of each steak. Brush with olive oil. Let stand while preparing the relish.
In a small bowl, combine the tomatoes, basil, onion, vinegar, and 1½ tablespoons oil. Season to taste with salt and pepper.
Warm a heavy large frying pan over high heat. Add the steaks and cook as desired, 3–4 minutes on each side for medium-rare. Transfer the steaks to a cutting board. Add the tomato mixture to the frying pan and cook until just heated through, stirring up any browned bits, about 1 minute.
Slice the steaks across the grain and divide among 4 warmed plates. Spoon the tomato relish over the top. Spoon the smashed potatoes alongside and serve right away.