In a medium bowl, add mineral water, melted butter, granulated sugar, lemon juice, vanilla extract and salt. Keep stirring until the sugar is dissolved.
Sift flour with baking powder, and add to the wet ingredients. Mix until well comined. Cover with cling film, and cool in the fridge for 20 minutes.
Divide the dough into two halves. In a greased springform (20cm or 9-inch wide), add the first half of the dough and spread it. Pierce with a fork. Followed by the apple filling, and the other half of the dough left. Pierce with a fork in a few places.
Bake in a preheated oven 180°C, and bake for 25-30 minutes until the top is golden.
Let the pie cool down completely, sprinkle with caster sugar and serve.