To make the crust, put the ground almonds, cornflour and sugar into a food processor and ‘pulse’ just to combine. Add the butter and blitz just until the mixture looks like fine crumbs. Tip the crumbs into the buttered tin and press evenly over the base and up the sides (right to the rim) using the back of a spoon – the crust will be about 5mm thick. Chill for 20 minutes.

Heat the oven to 180°C/350°F/gas 4 and put the baking sheet into the oven to heat up. Bake the crust for 12–16 minutes until a very light gold colour.

Meanwhile, make the topping. Set the lemons on a large plate (to catch all the juice) and trim off the ends. Cut each lemon into 8 wedges, removing the pips. Cut each wedge across in half and put into the food processor. Combine the cornflour and caster sugar and add to the processor bowl. Blitz just until the lemon is coarsely chopped. Add the egg and yolk and blitz briefly until combined. With the machine running, pour in the cooled butter through the feed tube. Stop the machine as soon as all the butter has been added – the mixture won't be smooth but should have a few visible tiny pieces of lemon.

Remove the crust from the oven and set it (in its tin) on the heated baking sheet. Turn down the oven temperature to 170°C/325°F/gas 3. Pour the lemon mixture into the hot crust to fill it almost up to the rim. Carefully return to the oven and bake for 30–35 minutes until the lemon filling is just starting to colour and is firm when you gently jiggle the baking sheet.

Remove from the oven and leave to cool completely, then cover lightly and chill for at least 3 hours, or overnight. To serve, run a thin, round-bladed knife around the inside of the tin to loosen the crust, then carefully unmould the tart. Dust with icing sugar and serve with whipped cream or ice cream. Store in an airtight container in the fridge. Best eaten within 2 days.