The previous day, pour the milk in a saucepan and add the elderflowers. Cover with clingfilm and let them infuse in the fridge overnight.
Make the shortcrust dough, too. In a large bowl, xix the flour with sugar and salt. Add the room temperature butter cut into cubes and rub it into the other ingredients using the tip of your fingers until you get fine crumbles, similar to grated Parmigiano Reggiano. You can also use a stand mixer with the paddle attachment.
Add a beaten egg and work again quickly, until the dough comes together. Flatten the dough into a disc and wrap it in a plastic wrap, then stash it in the fridge for a few hours, or better until the next day.
The next day, prepare the pastry cream. Bring the milk with the elderflowers to a simmer on low flame. When it starts to simmer, remove it from the heat.
In a saucepan, mix the sugar with the eggs, cornstarch and rice starch. Strain the milk through a sieve and then gradually pour it over the eggs, stirring constantly.
Bring it back on the stove and cook on the lowest flame, stirring continuously with a whisk, until the pastry cream thickens. Let it cool down completely, covered with a plastic wrap, before using it.
Preheat the oven to 190°C (375°F - gas mark 5).
Remove the shortcrust dough from the fridge, break it up in chunks and work it again on a floured work surface, then roll it out in a disk with a thickness of about 8 mm.
Grease with butter a 28 cm round tart pan with a removable bottom. Dust it with flour and line it with the shortcrust disc. Press it well on the bottom and all along the edges. Trim the excess dough. Prick the short pastry with the tines of a fork.
Line the shortcrust shell with a sheet of parchment paper and pour in some weights, such as dried beans, or a smaller pan, to blind bake it.
Bake the tart shell for about 15 minutes, then remove the beans and the parchment paper and bake for 10 more minutes, until golden.
Remove the tart pan from the oven, let it cool down for about ten minutes, then gently remove the tart crust from the pan. Set it on a wire rack to cool down completely.
Now prepare the strawberries. Wash them, remove the leaves and cut the larger ones into two or four parts. Collect them in a bowl and dress them with sugar and lemon juice. Also add the elderflowers. Marinate for about ten minutes.
When the pastry shell is cold, fill it with the pastry cream, smooth the surface, then arrange the strawberries on top. Drizzle their juice to finish. Let the cake rest in the fridge for at least an hour before serving.