1. Soak the onion in the balsamic vinegar with 1 tbsp cold water for 5 minutes. Rinse in cold water and drain well.
2. Meanwhile, slice each tomato into 4 horizontally.
3. Toss the onion, rocket and olives with 1 tbsp dressing.
4. Arrange some between each slice of tomato along with a little of the crumbled goats' cheese.
5. When each tomato has been re-formed, drizzle over the remaining dressing.