Heat oven to 190C/fan 170C/gas 5. Tip the flour and hazelnuts into a food processor, then add the butter. Process to fine crumbs, add the sugar, then mix briefly. Remove 3 rounded tbsp of the mixture and put in a separate bowl. Add 1-2 tbsp cold water to the food processor, then pulse to form a firm dough. Tip out onto a lightly floured surface and knead briefly. If the dough is soft, wrap in cling film and chill for 20 mins.
Roll out the dough and stamp out rounds using a 10-12cm cutter. You should get 12 rounds in total from the dough – you will need to gather and re-roll the trimmings. Use to line 12 x 7-8 cm tartlet tins or the 12 holes of a muffin tin.
Warm the jelly in a pan until it melts a little, then stir in the cherries. Divide between the pastry cases. Stir the chopped hazelnuts and sugar into the reserved pastry mix, then sprinkle over the cherries. Bake for 25-30 mins until the pastry and crumble are lightly browned and the cherries cooked. Leave to cool for 10 mins in the tins, then carefully remove – you may need to loosen the edges of the pies with a small, sharp pointed knife, then invert them onto plates and quickly flip the right way up. Serve warm or cold.