Roll out pastry to a square, 2mm thick. Cut into 4 equal squares. With the point of a sharp knife, score a line 1cm in around the edges of the squares - being careful not to cut all the way through. Brush around these ‘borders’ with beaten egg.
Thinly spread Grey Poupon Dijon mustard over the inside base of the pastries - not the borders. Place baby spinach leaves on top. Arrange Brie slices on top to cover. Finish with slices of sundried tomatoes, and tuck an extra spinach leaf in between the Brie slices.
Bake in a preheated oven until pastry is risen and crisp and cheese has melted, around 15 minutes. Delicious served hot - but great as a picnic treat served cold as well.