While the pie is cooking, prepare a caramel. First, place the remaining walnuts on a baking tray covered with baking paper (you can crush them a bit into large pieces).
Pour the remaining 100 g of sugar into a stainless steel pan and pour 6 cl of cold water over it, making sure to soak all the sugar.
Bring everything to the boil over low to medium heat.
Once the sugar has completely dissolved, watch for the first bubbles to appear.
Leave to cook for a further 3 minutes and turn off the heat to avoid burning the caramel.
Immediately pour the caramel over the walnut kernels, stirring a little if necessary, and leave to harden for about 20 minutes.
Then cover your tart with the caramelised nuts... and enjoy!