Pre-heat the oven to 200°C, Gas Mark 6.
In a small saucepan, melt the butter sugar, apricot jam and lemon juice over a medium heat. Reserve to one side.
On a lightly floured surface, roll out the pastry to a 12" x 8" rectangle. Transfer to a lined baking sheet and chill for 10 minutes. Brush with the beaten egg all over and arrange the nectarine slices on top overlapping each other in horizontal rows, making sure you leave a 2cm border all the way around.
Pour over the melted butter and sugar mixture and bake in the oven for 20-25 minutes until golden and risen. Serve immediately.