The general rule of thumb seems to be that you use the same as or a little more cream than you do lemon juice, and that the number of eggs you use are around 1 egg per 80-100ml of liquid (ie. the 50 per cent cream, 50 per cent lemon juice). However, some tarts call for whole eggs, some whole and extra yolks, and some just yolks. The more yolks you use the richer the tart will be; the more whites, the firmer the final tart. So if you're after a silky filling replace one of your eggs with two yolks. I find an all-yolk tart lovely in texture but a little too rich, and if made as a deep tart, it'll be a little soft and floppy - so I always make sure whole eggs account for at least 2/3 of my filling. You have a choice of adding wheat flour or cornflour to your filling. I prefer not to.
The base and sides of the tart can be brushed with a thin layer of chocolate once blind baked to stop the base going soggy. Or you can brush it with beaten egg white and bake until set - but make it thin - you don't want to create a white seal around the pastry. You can also add orange juice and zest to complement the lemon (but then it's citrus tart not lemon). With recipes that do not come from New Zealand, bear in mind cream can differ from country to country - in Britain we primarily use double cream (heavy cream in US cookbooks). This has around 48 per cent fat and can be boiled, whipped and poured. In NZ cream is generally 38 per cent fat. If you're using this your recipe will need a little more thickening from an extra egg yolk or two as the filling will be more watery. Also try adding more lemon zest and a little less lemon juice if you're short of eggs - the key is getting the water and fat content in a ratio that will allow the eggs to set it. Lemon juice is usually added last as it will thicken the filling. It is alright to whisk the sugar, eggs and lemon zest hard, but add the cream, and then the juice, slowly and gently.
My recipe would be thus for a 24cm blind baked pastry case:
- Whisk the finely grated lemon zest witheggs, egg yolks, vanilla extract and caster sugar till foamy, but not fluffy.
- Gently whisk in cream (look for a good rich one) then gently whisk in lemon juice.
- The tart case is always blind baked at 170-180 degC and should be golden before you add the filling.
- The filling is carefully ladled in, any bubbles that have formed from whisking it carefully scooped off. Bake at anywhere between 110 degC and 140 degC.
- The lower the temperature, the longer it needs to be cooked.
- If you warm the cream up when whisking into the eggs, lemon zest and sugar, it'll speed up cooking.