1. Preheat oven to 200°C (180°C fan-forced). Line a baking tray with baking paper.
2. Bring a large saucepan of water to the boil. Add beetroot and cook for 10 minutes, or until tender. Drain and return to the saucepan.
3. In a jug, whisk together vinegar, sugar and oil until the sugar dissolves. Pour dressing over beetroot in the saucepan and mix to coat. Using a slotted spoon, transfer beetroot to the prepared baking tray. Reserve dressing. Bake for 15 minutes, or until caramelised.
4. Meanwhile, place pan with reserved dressing over medium heat. Bring to a simmer. Simmer for 2 minutes, or until slightly thickened. Cool.
5. Layer rocket, onion and beetroot on a serving platter. Drizzle with cooled dressing. Serve sprinkled with feta and hazelnuts.
Tip: To slice onion very thinly, use a mandolin or V-shaped cutter.