Combine thyme, 2 tablespoons balsamic vinegar and 2 tablespoons oil in a glass or ceramic dish. Add steak. Turn to coat. Cover. Refrigerate for 30 minutes, if time permits.
Meanwhile, preheat oven to 200°C/180°C fan-forced. Line a large baking tray with baking paper. Place bread in a large bowl. Add 1 tablespoon oil and garlic. Toss to coat. Arrange, in a single layer, on prepared tray. Season with salt and pepper. Bake for 12 minutes or until golden.
Heat remaining oil in a large frying pan over high heat. Cook steaks for 2 to 3 minutes each side for medium or until cooked to your liking. Transfer to a plate. Stand, covered, for 5 minutes to rest. Reduce heat to medium. Add onion to pan. Cook for 8 to 10 minutes or until softened. Add semi-dried tomatoes, sugar and remaining balsamic vinegar. Toss until tomatoes are heated through. Transfer mixture to a large bowl. Add rocket, parmesan and croutons. Toss to combine.
Serve steak with salad.