Pour the lemon juice or lemon purée in a saucepan. Heat the saucepan.
Grate the zest of one lemon over the saucepan, using the Microplane grater.
In the meantime, whisk the whole eggs and transfer into a large bowl.
Add the castor sugar... and beat with a whisk.
Strain the soft gelatin on paper towel. (For better results, place the gelatine sheets in cold water one by one so they don't form a cluster.)
When the lemon juice starts boiling, pour it over the whisked eggs in two batches, while stirring with a whisk.
Transfer the preparation into the saucepan used to heat the juice... and combine vigorously with a whisk.
Cook over medium heat while whisking constantly so the preparation doesn't stick.
The preparation should quickly thicken. Remove from the heat as soon as you get a gentle boil... and add the white chocolate.
Combine well with a whisk. The white chocolate will melt quickly in the hot cream.
Add the strained gelatin and whisk until dissolved.
Pass the lemon cream through a fine sieve to remove any coagulated bits, zest or pulp.
Use as needed.