First, make the pastry. Sift the flour into a bowl and stir in the sugar and a pinch of salt. Add the butter and rub it in with your fingertips until you have a coarse crumb consistency. Stir in 2-3 tbsp of ice-cold water and bring the mixture together with your hands. Knead it against the side of the bowl, and if it comes together smoothly, squash it into a ball. If it’s still crumbling, add a little more iced water and work it until you have a pliable pastry dough. Wrap it in baking parchment and chill for 30 mins. Meanwhile, macerate the cherries in the sugar and lemon juice in a bowl.
To make the frangipane, blitz the almonds in a food processor until finely chopped, then add the butter, flour, vanilla extract and sugar, and blitz until combined. Add the egg, and blitz again to make a paste.
Heat oven to 160C/140C fan/gas 3. On a lightly floured surface, roll out the pastry into a large circle about 30cm wide, and the thickness of a pound coin. It doesn’t matter if the edges are split. Slide onto a baking sheet lined with baking parchment, and spread over the conserve, leaving about a 4cm border all the way around the edge. Cover this with the frangipane, then press half the cherries into it. Fold over the pastry edges to encase the edges of the cherries, pressing down the creases so it holds together. Brush with the beaten egg and scatter some granulated sugar over the cherries and pastry. Put in the freezer for 10 mins.
Bake the galette for 45 mins, then remove from the oven and allow to cool for 10-15 mins. Pour over the remaining fresh cherries and spoon over any juices. Serve with clotted cream, if you like.