Place the flour, butter, sugar and orange zest into a food processor and blend together.
Add the beaten egg until the ingredients come together.
Flatten the dough, wrap it in clingfilm and chill for at least 30 minutes.
Roll out (as quickly as possible because as it gets warm it becomes very difficult and sticky to handle) between two pieces of clingfilm and line the base of a 28cm tin (or smaller tins if you want individual tartlets). Any leftovers can be frozen for another time.
Place the tin(s) in the freezer for 10 mins or so and then bake blind using baking beans for 15-20 minutes until the pastry looks dry to touch.
Remove paper and beans and return to the oven for a couple more minutes. When cooked, leave in tin until you prepare the filling.
Mix the condensed milk, juice and rind of the lemons together.
Add the cream of tarter to the egg yolks and add to the lemon mixture. Pour into the baked pastry base(s).
Whisk the egg whites until peaks form, then whisk in the caster sugar a quarter at a time, whisking well.
The mixture should be firm and glossy. Spread or pipe the meringue over the lemon mixture trying to cover the pastry edges.
Bake in a cool oven 150C for about an hour until crisp and light brown.