Preheat oven to 180°C. Line a large baking tray with non-stick baking paper.Place the tomatoes on the tray in a single layer. If you can, turn them cut side up as this will help them cook better.Bake for 30 minutes until soft and lightly browned.Set the tomatoes aside to cool.Mix tomatoes, onions, sugar in a bowl with pepper and a small amount of salt.