Heat oven to 180C/fan 160C/gas 4. Tip
the biscuits into a food processor and blitz to
crumbs. Add the butter and pulse to combine.
Tip the mix into a fluted rectangular tart tin,
about 10 x 34cm (or 20cm round tin) and
press into the base and up the sides right
to the edge. Bake for 15 mins until crisp.
While the base is baking, tip the egg yolks,
sugar, and lime and lemon zests into a bowl
and beat with an electric whisk until doubled
in volume. Pour in the condensed milk, beat
until combined, then add the citrus juices.
Pour the mix into the tart case and bake
for 20 mins until just set with a slight wobble
in the centre. Leave to set completely, then
remove from the tin, cool and chill. Serve in
slices topped with thin lime slices, if you like.