Preheat the oven to 180C/350F/Gas 4.
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.)
Cook for 15-20 minutes, stirring regularly, until the butter has melted and the mixture has thickened.
Pour the mixture into the cooked pastry case , cover with clingfilm (to prevent a skin forming) and set aside to cool.
Arrange the orange slices on the cooled tart and sprinkle over the demerara sugar.
Using a cooks' blowtorch, heat the sugar until caramelised.
To serve, slice the tart and serve with double cream or custard.