Mix the flour, sugar and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Combine the egg yolks and water, add, mix quickly to form a soft dough – do not knead. Flatten the dough, cover and chill for approx. 30 mins.
Roll out the dough between two cut-open plastic bags to a thickness of approx. 2 mm. Cut out 8 circles each approx.10 cm diameter, place in the prepared tins. Prick the bases all over with a fork, leave to chill for approx. 15 mins. Cover the dough with baking paper, weight with dried pulses.
Blind bake for approx. 15 mins. in the bottom half of an oven preheated to 200°C. Remove the tins from the oven, allow to cool a little, carefully remove the tartlets from the tins and leave to cool on a rack. Place the tartlets on an oven tray lined with baking paper.
Place the eggs, sugar, lemon zest and juice and lime zest and juice in a thin-sided bowl and suspend over the gently boiling bain-marie, ensuring that the bowl does not touch the water. Reduce the mixture for approx. 6 mins. to a thick crème, stirring constantly, then stir in the butter, pour the crème through a sieve into a bowl, cover and chill for approx. 1 hr. Fill the tartlets with the lemon crème.
Beat the egg whites with the salt until stiff, add half of the sugar and continue to beat until the egg whites turn glossy. Stir in the remaining sugar. Pipe the egg whites onto the tartlets through a serrated nozzle.
Grill for approx. 3 mins. in the top half of an oven preheated to 240 °C (top heat or grill only).