Combine flour and salt. Work in butter, then add the sugar and egg yolk. Add 2 tablespoons cold water if needed and work into a dough. Wrap in cling film and chill for 30 minutes.
Preheat oven to 200 C / Gas 6.
For the tart filling:
Peel, core and chop 6 apples. Place in a pan, add 1/2 of the lemon juice and 2 tablespoons water, cover and cook for 15 to 20 minutes, stirring occasionally until tender. Add 1/2 of the sugar and all of the butter and stir into a pulp.
Roll out pastry, and use it to line a 25cm (10 in) loose bottomed flan tin. Prick the pastry and top with a sheet of baking parchment, fill with a couple of handfuls of dried beans or rice and bake blind in the preheated oven for 10 minutes. Remove rice and parchment, and return to oven for another 5 minutes.
Reduce oven temperature to 180 C / Gas 4. Peel, core and slice 2 apples, toss in remaining lemon juice and sugar. Spread apple puree over pastry, place slices of apple on top.
Bake for 30 minutes. Cool slightly, brush with warmed sieved jam.