Grease a rectangular tart tin with a removable base. Press Borgs puff pastry into the tart tin and blind bake it at 160C for 10 minutes. Cool.
In a large pot heat the oil and on a low heat, sauté the onions until they are sweet and translucent. This should take about 30 minutes. Remove from the heat and transfer to a mixing bowl. Cool slightly.
Add the egg yolks, cream and season with freshly ground pepper and salt.
Pour the onion mixture into the tart tin.
Bake at 180C for 25 minutes or until the tart is set and the top of brown and delightful!