On a lightly floured work surface, roll out the pastry until it is about ⅛-inch in thickness. Line the prepared tart tin with the pastry, making sure not to stretch it. Roll your rolling pin over the top to trim any excess and prick the base of the pastry all over with a fork. Chill in fridge for at least 15 minutes. This helps the pastry relax and reduces the chance of it shrinking when you bake it.
In a standing mixer fitted with a paddle attachment, or with a handheld mixer, cream together the butter and sugar until pale and fluffy. Beat in the eggs one by one. With a spoon, stir in the almonds and flour and mix until just combined. Spread over the chilled tart base and then top with the quartered plums in in any pattern you like.
Bake the tart for 10 minutes and then reduce the heat to 350°F and continue to bake for another 25-30 minutes. Remove from oven and allow to cool completely. Dust with confectioner’s sugar before cutting and serving.