Halve, stone and quarter the plums and put to one side. Heat the butter in a large pan with the spice and muscovado sugar for 1-2 minutes, until the butter has melted. Tip in the plums, zest and port and cook for 1-2 minutes, stirring until the sugar has partially dissolved. Remove from the heat.
Preheat the oven to 200C/gas 6/fan 180C. Roll out the pastry on a lightly floured surface and trim to a rough 38cm round. You don’t have to be too neat. Lift up the pastry by draping it over a rolling pin and then laying it over a large baking sheet. The pastry should be too big, with a lot hanging over the edges.
Sprinkle the ground almonds over the pastry to within 7cm of the edges. Pile the plums and their juices over the almonds, then flip the overhanging pastry over the fruit, leaving a big hole in the middle. There’s no need to be too fussy – as long as the steam can escape and the fruit shows, it will be fine. Brush the pastry with the beaten egg and sprinkle the demerara sugar over the top. Bake for 30-40 minutes until golden.
A few minutes before serving, gently heat the custard in a pan, then stir in the marsala or sherry. Serve the pie with lingering warmth and the custard.