Knead the flour together with sugar, butter, egg and salt until obtaining a smooth dough.
Cover it with some kitchen paper and place it in the fridge to rest for 30 minutes.
Then take it out of the fridge, place it on a sheet of baking paper and roll it into a 1.5 cm thick round shape. Put the dough into a round 23 to 25 cm springform pan and press it down. Make sure that you have at least 3 cm of extra dough at the sides.
In the meantime, peel the apples, cut them in quarters, remove the core and slice each quarter in 4 or 5 pieces. Place the apple slices on top of the dough. Make sure to put them all in the same way and go round and round. Press the sides of the dough onto the apples to make a nice border.
Preheat the oven at 180° fan forced and bake the apple tart for 40 minutes or until golden. When ready, let it cool.
In the meantime, dissolve the tortagel and the sugar in water and boil the mixture for 1 minute. Let the jelly cool down for 2 minutes and then pour it on top of the tart. Let it set in a cool place for at least 30 minutes before serving it.