Pre heat oven to 180c/350f (fan forced).
Using a food processor (or a sturdy zip lock plastic bag & a rolling pin) process biscuits into fine crumbs, then add melted butter to combine.
Line the base of a 25cm/9 1/2 inch loose bottom tart tin with baking/parchment paper (see notes).
Press biscuit mixture firmly into the base and sides of the tin.
Place tart tin on a baking sheet, and bake for 10 mins, then allow to cool.
While base is cooling, in a medium bowl combine lemon curd, egg & sugar, mixing until smooth.
Add cornflour & mascarpone, again mixing until smooth.
Place raspberries evenly over the cooled biscuit base, then pour mascarpone mixture over the top and smooth out the mixture - being careful not to move the raspberries around too much (see notes).
Bake for 30 mins - tart should be set in the middle but with a slight wobble, and light brown around the edges.
Allow to cool for 10 - 15 mins, then refrigerate for 2 -3 hours to set.