2.Beat flour, butter, sifted sugar and salt in a bowl with an electric mixer on low speed until a fine texture. Beat whole eggs in a bowl with a fork; reserve a quarter of the beaten egg for brushing. Add remaining egg and egg yolk to flour mixture; beat on high speed for 1 minute or until a dough forms. Shape into four discs; wrap each with plastic wrap. Refrigerate for 25 minutes.
3.Make apple puree and sugar glaze.
**Apple puree** Combine ingredients in a heavy-based saucepan over medium heat; cook, covered with a tight-fitting lid, for 15 minutes or until apples are soft. Cool slightly. Blend apples with a hand-held blender until smooth. Cool.
**Sugar glaze** Stir icing sugar and juice in a small bowl until smooth.
4.Roll each pastry disc between sheets of baking paper into 22cm (9in) rounds. Transfer on their paper to two large oven trays. Refrigerate for 30 minutes.
5.Slice each apple into 1mm (⅛in) thick rounds with a mandoline (use one apple for each tart).
6.Spread apple puree on pastry rounds, leaving a 4cm (1½in) border. Overlap apple slices on puree, starting from the outer edge and working around towards the centre. Brush apple well with sugar glaze.
7.Gently fold in the pastry edge every 5cm (2in). Brush pastry edge lightly with reserved egg. Bake tarts for 30 minutes, rotating trays halfway, or until golden; cool.
8.Make tarragon meringue; top tarts with meringue. Using a blowtorch on medium, lightly brown meringue.
Serve tarts topped with extra tarragon leaves, if you like.
9.Stir sugar, the water and tarragon in a saucepan over medium heat, without boiling, until sugar dissolves. Discard tarragon. Bring syrup to the boil; boil, without stirring, for 10 minutes or until temperature reaches 112°C/235°F on a sugar thermometer (see tip). Meanwhile, whisk egg whites and cream of tartar in a bowl with an electric mixer on low speed for 5 minutes or until frothy; whisk until syrup is ready. When syrup reaches temperature, increase mixer speed to high; gradually pour syrup into egg white mixture. Continue whisking until mixture is thick and glossy.
Drop a little syrup into a glass of cold water. Gather the syrup up; when it forms a soft, flexible ball, the syrup is ready to use.