Heat an oven to 170C. Line the tart tin with the pastry and use a fork to prick holes in the base. Bake blind by covering with baking parchment and rice/pulses/baking beans and bake for 15 – 20 minutes. Take out and carefully remove the parchment and weights and then return to the oven, turning the temperature down to 150C. If any cracks appear, use some spare pastry to patch them. Bake for a further 10 – 15 minutes, or until the pastry is golden brown all over. Take out and allow to cool on a wire rack
Put the cream and the fennel seeds in a pan and carefully bring up to the boil, stirring frequently, to avoid the bottom burning. Turn the heat off and allow to infuse for 30 minutes, then using a sieve to remove the fennel seeds, strain into another pan.
Set the oven to 130C. Put the tahini in large jug. Combine the egg yolks and sugar in a bowl and whisk till the volume increases by one third. Warm the cream again, then slowly add whisk into the egg mixture. Return this mixture to the pan and cook on a low heat, stirring with a spatula constantly, until the mixture thickens enough that you can draw a line across the back of the spatula. Decant into the jug with the tahini and stir thoroughly but not vigorously. Add the preserved lemon and mix.
Put the tart shell on a tray and place in the oven. Carefully pour the custard mixture into the tart shell whilst it is on the oven shelf, or rack. Bake for 15 -20 minutes and then check to see if it’s ready by giving the tart a little shake. You want it to be set but still to have some wobble. If ready, take out and allow to cool before serving. If not, cook for another 5 minutes, then check again.