If the filling cooks fast and gets chunky and set at the edges of the bowl, just
give the mix a vigorous whisk off the stove to redistribute the heat.

Pre-cooking the filling gives you a creamier mouthfeel and means the
custard won’t separate into layers during baking.

Remove the bowl from the heat and pour the filling through a fine
sieve into a measuring jug. Discard the zest. If you have a little froth
on the top you can spoon this off, but it’s not vital. Set the filling aside
at room temperature while you prepare the crust.

Preheat your oven to 120°C (250°F) and adjust the oven racks – set
one in the lower part of the oven and remove all the ones above it,
allowing space to manoeuvre a jug in easily to pour the filling in later.

See Don’t leak: how to win at liquid-filling tart life (page 94).

Place the blind-baked crust, still in the tin, on a shallow baking
tray. If there are any large cracks or dipped sides, massage a little
leftover dough to a soft paste consistency and gently patch any large
fissures – taking care not to press hard and break the crust. Then
meticulously brush the inside of the tart with the whisked egg white.

Return the baking tray and crust to the oven and bake for 3 minutes
to seal the crust before filling. With the tart still in the oven, and your
eye on where the crust edge is lowest, carefully and slowly pour the
custard into the crust to fill as far as you can. If your jug, when tilted,
is too big for the oven, switch to a smaller cup to scoop in the last of
the mix. Don’t overfill, or the filling will cascade between the crust
and tin.

I have given you a little extra filling in case you need it (slightly larger tin,
super tall crust sides with no blind-baking shrinkage – you genius!). You can
cook the leftover on the double boiler to 80°C (175°F), then chill to set in a
wee dish. Eat with cream.

Bake for 40–50 minutes (I always check after 40, and then check in
5-minute increments) until there is a 7 cm (2¾ in) diameter centre
of quivering, thickshakey filling in the middle of the tart. This wobbly
centre will continue to cook and set firmer upon cooling. Turn the
oven off, keep the door closed and leave for 5 minutes. The wobbly
centre should have set firm and feel like the most delicate bouncy
custard trampoline when touched lightly with your fingertips. Leave
another 5 minutes if you think it’s still a bit too wobbly.*

I try to avoid breaking the surface with a probe thermometer but if you need
the assurance, the read should be 70°C (160°F) internal before you turn the
oven off to rest the tart.