Preheat oven to 375°F (190°C). Lightly grease a 9-inch tart pan with removable bottom.

Prepare the crust: In a medium bowl, combine flour and sugar. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Mix in the egg yolk until a dough just forms. Press the dough evenly into the bottom and up the sides of the prepared tart pan.

Partially bake the crust for 12-15 minutes until lightly golden. Remove from the oven and let cool slightly while preparing the filling. Reduce oven temperature to 350°F (175°C).

Prepare the filling: In a separate bowl, beat the softened cream cheese and sugar until smooth. Beat in the egg, vanilla extract, and lemon zest until just combined, taking care not to overmix.

Pour the cream cheese mixture into the warm crust. Arrange the thinly sliced nectarines decoratively over the top of the filling, overlapping slightly.

Bake for 30-35 minutes, or until the center of the filling is mostly set (it should still have a slight wobble). Let cool completely on a wire rack.

For the glaze, gently warm the apricot jam with 1 teaspoon of water in a small saucepan until melted and smooth. Brush the warm jam lightly over the top of the cooled nectarines for shine. Chill for at least 2 hours before slicing and serving.