For the brioche dough, place the flour, salt, sugar and yeast (keeping the yeast away from the salt as it will attack it and damage its ability to ferment), in an electric mixer bowl. Add the eggs and mix with a dough hook attachment for 5 minutes on low power until the eggs are completely incorporated (alternatively, place the ingredients in a large mixing bowl and stir together for 5 minutes).
Increase the speed of the machine or your stirring and mix for another 5 minutes until the dough comes away from the edge of the bowl. Then add the cubes of butter and continue to mix for 2-3 minutes until completely incorporated.
Remove the bowl from the machine, if using, then cover with clingfilm and set aside at room temperature for 1 hour to prove, then chill the dough for a further hour (it will be easier to work with).
Preheat the oven to 190C/170C (Fan)/Gas 5.
Lightly flour a work surface and your hands. Bring the dough together with the palms of your hands to form a ball, then place it on a baking tray and flatten it slightly. Starting from the middle of the dough, gently press the dough flat and spread it out to form a circle to approx 23cm/9in in diameter, but leave a 2cm/1in gap from the edge as this will create the rim of the tart. Be careful not to stretch the dough and try to keep the base even in thickness.
Cover with a clean tea towel and place the dough in the warm area for 25 minutes.
For the crème filling, mix the egg yolks, sugar, lemon zest and juice together in a large mixing bowl and gradually mix in the crème fraîche. Set aside.
For the glaze, brush the rim of the gateau with the egg yolk and prick the base of the dough evenly with a fork to help the even cooking and rising of the dough. Pour the crème mixture over the dough, sprinkle with the caster sugar and dot with the butter, Sprinkle with the nibbed sugar and bake in the preheated oven for 25 minutes, or until the brioche has risen and the filling is set. Remove from the oven and set aside to cool, then serve.