1. Heat the rapeseed oil in a saucepan over a low heat and cook the onions for 25 – 30 minutes.
2. Pre-heat oven to 150°C / 300°F / gas mark 2.
3. Line a baking tray with lightly oiled baking paper.
4. Lay the pre-rolled puff pastry on the tray cutting off any bits that go over the edge.
5. Thinly spread the caramelised onion on the pastry leaving a 2cm edge.
6. Thinly slice the tomatoes and lay over the onion.
7. Bake for 1 hour. Check to make sure pastry does not burn.
8. Remove from oven when the pastry is golden
9. Roughly tear and sprinkle the fresh basil over the tart
10. Season with salt and pepper to taste