Add the cashews, tofu, lemon zest, and elderflower liqueur to a high-speed blender.
Add the lemon juice, evaporated cane sugar, salt, and agar powder to a small saucepan. Bring the mixture to a boil, then reduce to a simmer for about five minutes, stirring frequently.
Immediately pour the mixture in the saucepan into the high-speed blender. Blend on high until completely smooth.
Immediately pour the filling over the crust. Allow to set in the fridge until completely firm.