Sprinkle steak with 1/2 teaspoon salt and 1/4 teaspoon pepper. Grill, covered, over medium heat or broil 4 in. from heat 6-8 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Let stand 5 minutes., Meanwhile, in a small bowl, whisk oil, vinegar, lemon juice and the remaining salt and pepper. Divide spinach among 4 plates. Add tomato, avocado and radishes., Cut steak into slices; place over salad. Drizzle with dressing; if desired, sprinkle with cheese.