Mix the flour, powdered sugar, softened butter, grated lemon zest and salt and knead with your fingers to form a dough. Mix in the egg. Shape the dough into a ball, wrap in cling film and put in the refrigerator for 1-2 hours.
Roll out the dough in a tart pan. Prick several times with a fork, cover with dried fruit or weights and bake for 10 minutes at 200 ° C. Remove the weights and let them cool.
In a bowl, whisk 2 eggs and 2 egg yolks with 200 g powdered sugar.
Rub 1 of the 3 lemons, squeeze out all the lemons and the orange and mix the juice with the eggs, lemon peel and melted butter. Pour onto the prepared base and bake at 180 ° C for 20 minutes.
Beat the egg whites with 100 g powdered sugar until stiff, distribute on the tart and bake until the meringue top is lightly browned. The tart can be served lukewarm or well chilled.