Heat grill pan over medium-high heat. Brush steaks with oil; sprinkle lightly with salt and pepper. Cook steaks for 8 to 12 minutes or until desired doneness (160 degrees F for medium), turning once halfway through cooking. Transfer to cutting board; cover and let rest for 2 minutes.
Meanwhile, divide salad greens and radishes among four plates. For dressing, in a bowl combine yogurt and mint; season to taste with salt and pepper.
Remove bone from lamb. Cut lamb in strips; place meat on salad greens. Sprinkle with feta; pass yogurt-mint dressing. Serves 4.