By Tonia Buxton
This tart is such a lovely showstopper. I make it when I’m hosting a drinks party because it’s as delicous as it is beautiful.
Preheat the oven to 190°C/375°F/Gas Mark 5.
Place half the butter and half the oil with the onions and sugar in a heavy-based pan and caramelise on a medium heat until the onions have darkened. Remove from the pan and set aside.
Add the remaining butter and oil to the p