Roll out the pastry on a lightly floured work surface. Use the pastry to line a 23cm loose-bottomed tart tin (see step-by-step). Cover with cling film and leave in the fridge to rest for 1 hr.
Heat oven to 190C/170C fan/gas 5. To make the filling, put the butter, sugar and lemon juice in a food processor, then blend until light and fluffy. With the machine running at full speed, add the eggs and almond extract, and whizz until combined to a smooth paste. Fold in the breadcrumbs and ground almonds by hand.
Take the tart case from the fridge, line with baking parchment and baking beans (see step-by-step) and place on a baking tray. Bake for 20 mins until the sides are set and the pastry is sandy (see steps). Carefully remove the beans and the parchment and place tart case back in the oven for 10 mins until the base is biscuity.
Remove from the oven and spread the jam over the base, then top with the almond filling. Return the tart to the oven for a further 25 mins until the almond layer is cooked. Leave to cool in the tin.
Remove the tart from the tin and place on a serving plate. Mix the icing sugar with 4-5 tbsp water to make a thick icing. Flood the top of the tart with the icing, keeping 1 tbsp aside, then arrange the raspberries on top, all pointing upwards. Drizzle with the remaining icing and scatter with flaked almonds. Leave for 10 mins so the icing can set a little, before serving.