Place the biscuits and walnut kernels in a bowl. Mix in the butter and fleur de sel, spread the mixture over the prepared base, press down firmly using the back of a wet spoon, chill.
Combine the yoghurt, sugar, the lemon zest and lemon juice. Dissolve the gelatine in hot water, combine together with 2 tbsp of the yoghurt mixture and immediately stir well into the remaining mixture. Spread the yoghurt mixture over the biscuit base, cover and chill for approx. 4 hrs.
Mix the rhubarb and sugar in a pan and leave to absorb for approx. 30 mins. Add the water, bring to the boil. Cover and simmer for approx. 10 mins. until soft, puree, allow to cool a little. Using a whisk, combine the rhubarb puree, sugar, cornflour, egg yolks and butter in a pan and bring to the boil over a medium heat, stirring constantly. While hot, pass half of the creamy mixture through a sieve into a clean, pre-warmed jar, seal immediately. Stir the gelatine into the rest of the hot mixture, then pour the mixture through a sieve into a bowl, allow to cool, stir until smooth. Spread the curd over the yoghurt layer, chill for approx. 2 hrs. more. Remove from the tin, cut diagonally into 8 slices. Scatter the blossoms over the top.