Knead butter, coconut oil, flour and salt with the dough hook of the hand mixer to a crumbly mass. Then add four tablespoons of ice water and knead to a smooth short pastry (possibly add another one or two tablespoons of ice water if the pastry is too crumbly).
Wrap in foil and refrigerate for two to three hours. Squeeze the lemon and put it in 1/2 litre water. Peel and quarter apples, remove the core and cut the apple quarters into slices.
Place in the lemon water. Roll out the short pastry between two layers of cling film or on a floured work surface two centimetres larger than the tart (26 centimetres diameter).
Place in the form, press on and cut off the excess dough. Prick the base of the dough several times with a fork. Drain the apple slices well and lay them on the dough in flakes. Heat up the jam and brush the apple pieces with it. Bake in a preheated oven (electric range: 225 ° C/ gas: level 4) for 25 to 30 minutes (cover if necessary in between).
Serve lukewarm dusted with icing sugar. Makes twelve pieces.