Nutrition: Per serving
First, make the pastry. Put the flour in the bowl of a food processor. Add the butter, yeast extract and a good pinch of salt. Whizz until you have a crumbly mixture that starts to clump. Add 1 tbsp water at a time, whizzing the mixture as you do (we used 2 tbsp water in total). As soon as the dough comes together, stop the processor and don’t add more water.
Tip the dough out onto a work surface and lightly roll out to the thickness of a £1 coin. Wrap and chill for 30 mins.
Remove the dough from the fridge, then roll out on a lightly floured work surface until large enough to line a 20cm fluted (4cm high), loose-bottomed tart tin, with some overhang. Carefully use the rolling pin to lift the dough up and lay it into the base of the tin. Use your hands to encourage the pastry into the base and up the side of the tin, gently pushing it into the fluted edges. Leave any excess overhanging the rim. Put in the freezer for 15 mins and heat the oven to 180C/160C fan/gas 4 with a baking tray on the middle shelf.
Remove the pastry case from the freezer and prick the base all over with a fork. Line the inside of the case with a sheet of scrunched-up baking parchment, pressing it into the base and up the side. Pour in baking beans or dried rice or lentils to weigh the parchment down.
Bake on the hot tray in the oven for 25 mins, then remove the parchment and baking beans and trim off the excess pastry using a small serrated knife. Return to the tray in the oven and bake for another 10 mins. Meanwhile, mix the eggs and the milk together in a jug with some seasoning until combined.
Remove the tart case from the oven and transfer to a wire rack. Spread the piccalilli over the base, then sprinkle over the cheese and carefully pour over the egg mixture. Sprinkle with the onion seeds.
Return the tart to the baking tray in the oven for 30 mins until the filling is set with a slight wobble in the centre. Leave to cool completely in the tin. Chill until cold, then cut into wedges to serve. Will keep chilled for up to three days.
