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.
Preheat oven to 190C/375F. Lightly grease a bun or muffin tray. .
Take half of the dough from the fridge and roll it out thinly. Cut circles slightly larger than the holes in the tray using a cookie cutter or the edge of a glass. Cut out shapes, such as stars, to top the mince pies. Gently press the circles into the tray, fill ¾ full with the mincemeat and top with the star shaped pastry.
Bake for 10 minutes or until they just begin to brown. Remove from the tray and cool on a wire rack. When the tray is cool, repeat with the remaining pastry.