Prepare dressing by whisking together oil, vinegar, salt and pepper. Brush each side of steak with dressing and sprinkle with thyme and rosemary. Grill steak and onion slices. Allow steak to rest for ten minutes and slice into 1/4 inch slices. Divide salad onto two large plates. Top with grilled steak, onion, tomato and cheese. Pour dressing over all.