Make the pastry first. In a food processor, whiz the spinach until fine. Set aside.
In the food processor (no need to clean out too thoroughly after the spinach), pulse the flour and butter together until it resembles breadcrumbs. Add the sugar, egg yolk and spinach and pulse gently until it forms a ball. You may need to add a tablespoon or so of cold water. You don’t want it too wet.
Press the dough into a flat disc shape, to aid rolling later, cover in clingfilm and refrigerate for at least half an hour or even overnight.
Preheat the oven to 190C/375F
For the frangipane:
Cream the butter and sugar together. Add in the eggs, one at a time, and beat well, then add the almond extract. Gently stir the ground almonds and flour in manually, being careful to combine by not over mix.
For the filling:
Slice the nectarines thinly. In a bowl combine the nectarines, sugar and flour and gently mix. Set aside.
Roll the pastry into a large round, about 12” diameter, on a large piece of lightly floured baking parchment. Transfer the pastry and parchment to the baking sheet.
Spread the frangipane paste on the centre of the pastry, leaving 2 inches around the edges of the circle. Place the fruit in concentric circles over the frangipane. Fold the edge of the dough over the fruit, pleating rustically. Brush the edges with milk, sprinkle the sugar over the fruit and dough and bake for 45-55 minutes until cooked and lightly browned.
Allow to cool slightly on the baking tin, and then transfer the galette, along with the parchment to a wire rack to cool.