1. To make the pastry, dice the butter, then pulse with the flour in a food processor until the mixture resembles fine breadcrumbs. Add the sour cream and continue to pulse until the dough starts to incorporate into a ball.
2. Wrap the dough in plastic film and refrigerate for 20 minutes.
3. Roll the chilled pastry out to 2mm thickness and carefully line tart tin. If blind baking for a tart, gently press the pastry into the edges and trim off the excess pastry 5mm above the top of the tin, this will allow for shrinkage.
4. Line the pastry with baking paper and fill with baking beads. Dried beans, chickpeas or rice will do the trick if you don’t have baking beads – just don’t use them to cook with later!
5. Place into the preheated oven 220°C and bake for 15 minutes, then remove the baking beads and baking paper and continue to bake for another 10 minutes. Remove and set aside to cool, ready for your filling.
6. Turn the oven down to 140°C.

Filling

1. Whisk the sugar, orange juice and zest, orange blossom water, eggs and egg yolks together in a bowl until well combined. Add the butter and set over a pan over simmering water. (Do not let the base of the bowl touch the water.)
2. Cook for 15-20 minutes, stirring regularly, until the butter has melted and the mixture has thickened. This may take longer, keep an eye on the mixture and wait until you can draw a line in it on the back of a spoon – a little like making Anglaise or custard.
3. Pour the mixture into the cooked pastry case and return to oven for 30 minutes at 140°C
4. Arrange the orange slices on the cooled tart and sprinkle over the demerara sugar.
5. Using a cooks’ blowtorch, heat the sugar until caramelised.
6. To serve, slice the tart and serve with double cream or custard.