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Put the flour, half the hazelnuts, the icing sugar and butter in a food processor and blitz to a sandy texture. Add the egg yolk and 1-2 tbsp water, and blitz briefly until the dough clumps together. Tip out onto a work surface and knead the dough a little until smooth. Flatten to a puck shape, wrap in cling film, then chill for 30 mins.
Mix the mincemeat, apple and orange zest in a bowl. Heat oven to 200C/180C fan/gas 6. Unwrap the pastry and roll out on a lightly floured surface to the thickness of a £1 coin. Stamp out 12 circles with a 10cm fluted biscuit cutter and use them to line a 12-hole muffin tin.
Fill each pie with the mincemeat mixture. Scrunch up the pastry trimmings and re-roll to the same thickness as before. Stamp out 6 circles using an 8cm cutter, and 6 stars. Put the remaining hazelnuts in a dish and lightly whisk the egg white in another. Brush a little egg white around the inner rim of each pie, then brush one side of each pastry lid, and the stars. Press each lid into the hazelnuts, egg-side down, so they stick, then lightly press on top of the pies, nutty-side up. Repeat with the stars and place these on the remaining pies.
Bake for 20 mins or until golden and crisp. Leave to cool in the tin for 5 mins, then scoop out and cool on a wire rack. Dust with icing sugar before serving. Will keep in a sealed container for 3 days.