In a mixing bowl, using your hands, rub together the flour, butter, icing sugar, lemon or lime zest and pinch of salt – rub in till it resembles breadcrumbs, as they always say.
Now add the beaten egg yolk and mix well with a spoon, adding water as needed: it should be a nice, soft dough that comes together, but don’t add too much water as you want a crumbly, buttery pastry.
Once it forms a soft dough, wrap in clingfilm and chill in the fridge for 35 minutes or so.
Preheat the oven to 180ºC (160ºC for a fan oven). Take a 6-hole or 12-hole tart-baking tray (I use small Yorkshire pudding trays) and grease it with butter.
Flour the work surface lightly and roll out the pastry to about 2–3mm thick. Now cut out circles big enough to fill the tart holes nice and neatly to the rim, and tuck them in. (You may have dough left over – freeze it for future use.)
Line the tartlets with baking paper, put some baking beans on top, and bake for 13 minutes. Then remove the beans and baking paper, and bake for another 4–5 minutes.
Finally dollop your desired jam on the tartlets – enough to cover the bottom, about a tablespoon for medium-sized tarts – and bake for about 10 minutes, until the jam begins to bubble.
Leave to cool down, then remove them while slightly warm from the tray and eat.