Put the butter and flour into the bowl of a food processor, then pulse until the mixture resembles coarse breadcrumbs. Add the sour cream and pulse again until the dough just forms a ball.

Form into a flat disk, cover with cling film and refrigerate for 20 mins.

Roll dough out to fit a round tart pan 25cm x 2.5cm (10” x 1”), using a fork poke holes all over the pastry and then fit the pastry into the tin and chill again for 60 minutes.

Preheat oven to 200c (390f) bake not fan.

Trim the excess pastry from the tin using a sharp knife.

Line the pastry case with a double layer of foil and fill with sugar to the top of the tin.

Blind bake the pastry case for 15 minutes, then remove the foil and sugar. Return to the oven for another 5 minutes or until the pastry is pale golden and the bottom is dry.

While pastry is blind baking prepare the lemon mascarpone mixture.

Reduce the oven temperature to 170c (340f).

Place ricotta and mascarpone into a food processor and process until smooth.

Add sugar, eggs and yolks, lemon rind and juice and process again.

Put the tart tin on the middle oven shelf.

Carefully pour the mixture into the pastry.

Bake for 30 minutes or until just set. There should be a slight wobble to the centre when you jiggle the tin.

Remove from oven and allow to cool in the tin, then refrigerate still in the tart tin (mixture thickens up as it cools).

Remove from tin and serve.

Put water and sugar into a frypan over medium heat and cook until mixture starts to turn golden brown.

Slide in lemon slices and cook for a couple of minutes then turn slices over.

Cook until slices are well coated with the toffee.

Remove slices and place on a sheet of baking paper to cool.

Arrange slices over the top of the tart.