Make the pâte sucrée: Process flour, icing sugar and a pinch of salt in a food processor to combine, add butter and iced water, process until mixture resembles fine crumbs. You can also do this very quickly with a pastry cutter. Add yolk, process to combine, turn onto a work surface and bring together with the heel of your hand. Divide the dough into 2 balls and form each dough into a disc. Wrap each disc in plastic and refrigerate, for about 1 1/2 hours.
Make curd filling: Place all the ingredients in the double boiler over simmering water. Ensure that the top pan does not touch the water. Cook and continuously stir until mixture begins to gel or thicken, for about 15-20 minutes (yes, tough job but it will be worth it, trust me). Remove from heat, transfer to a bowl and allow to cool slightly. cover with cling wrap touching the surface of the curd and refrigerate it to thicken more. The curd keeps, covered and refrigerated, for about one week.
Bake the tart shells: Preheat oven to 180°C/350°F/gas mark 4 (if using convection, turn the fan on). Working with one dough disc at a time, divide dough into 8 pieces. Roll out each piece on a lightly floured surface to 2mm thick and line 8 3-inch-diameter, 3/4-inch-deep fluted tart shells. Trim edges and refrigerate to rest, for about 30 minutes. Collect remaining pastry into a ball and return it to the fridge. Repeat with the second dough disc. Combine the leftover dough and repeat to line remaining tart shells till dough it used up. Blind bake tart shells (fork, line and weight) until light golden, for about 15-20 minutes. Remove paper liners and weights, brush shells with egg wash. Bake again until crisp and golden, about 4-5 minutes (if using convection, turn the fan off). Remove pastry shells from tart tins and cool on a wire rack. Baked shells keep well in an air-tight container, with layers of parchment in between, at room temperature, up to 3 days.
Make the Italian meringue: Meanwhile, remove the lime curd from the fridge to soften slightly. To prepare the Italian meringue, combine sugar and water in a small pot over medium-high heat. Clip a candy thermometer onto the pot. Without stirring, bring the syrup to a boil and let it cook gently until the temperature reaches 240°F. Meanwhile, place the egg whites and cream of tartar in a clean bowl of a stand mixer. With the whisk attachment, begin whisking the egg whites when the syrup just begin to bubble on the stove. Start whisking on medium low until the whites begin to foam all over, then increase speed to medium high. Whisk until medium stiff peaks form and stop the mixer.
Once the syrup has reached 115°C/240°F (soft ball stage) and the whites have been whipped to soft peaks, restart the mixer and gradually pour the syrup in a small stream into the egg whites, whisking on medium high continuously. Be careful not to allow the sugar syrup to fall on the beaters. Continue whisking until the bowl of the mixer is at body temperature and not warm to the touch anymore. (This took me a good 15 minutes.) Meringue keeps well in a sealed piping bag at room temperature for at least a few hours.
Assemble the tarts: If you have the time flexibility, do this as close to serving as possible. This keeps everything fresh and the tart pastry shells crispy. Line each tart shell with about 8 pieces of raspberry halves, if using. Spoon over lime curd to fill the tarts. Fill the meringue into a piping bag attached with the nozzle of your choice. Pipe meringue over the tarts to cover partially or completely. Brown the meringue with a blow torch (if you're using the broiler, cover the tarts completely to avoid toasting the filling). If covering partially (like I did), sprinkle the uncovered half of the tarts with lime zest and top with small sprigs of red currants or any other garnish(es) of your choice.
Do ahead: While the recipe reads pretty long, two of the components can be made ahead. I used leftover lime curd from about a week ago. Just bring it to room temperature from the fridge and give it a good stir (folding in method) with a spatula to soften and smoothen. The tart shells were made the day before. Only the meringue has to be prepared the day of, which really it possible for me to show up calm and collected for that fantastic dinner.