In a small bowl, whisk together vinegar, mustard and salt. Gradually whisk in olive oil.
Heat grill pan on the stovetop or set grill to medium-high heat. Coat cooking surface with nonstick cooking spray. Season steak with Greek seasoning. Grill for 4 minutes. Turn and grill 4 to 6 minutes more or until internal temperature reaches 135 degrees F on an instant-read thermometer, for medium-rare. Allow to rest for 5 minutes. Thinly slice against the grain.
On a large serving platter, fan out tomatoes. Scatter cucumber, radishes and feta over the top. Drizzle the dressing over the salad and serve with the sliced steak.