Melt butter in a small saucepan over medium heat. Dissolve the brown sugar in the butter.
Add the pears, cardamom and a pinch of salt. Lower the heat the medium-low and cook the pears for 4 to 5 minutes until they're tender. With a slotted spoon, remove the pears to a bowl, leaving the cooking liquid in the saucepan.
Add a splash of balsamic vinegar to the cooking liquid. Simmer it for about 3 to 4 minutes until it reduces by about half. Watch it carefully as it will bubble.
Preheat the oven to 400°F.
Unfold the puff pastry and cut it into 4 equal squares.
Using a very sharp knife or pizza roller, about 1 cm in from the edge, make a cut from each corner along the edge to almost the halfway point. Repeat this on each corner and to each square of pastry (see photo).
Divide the cooked pears between the 4 squares of pastry. Crumble ¼ oz of Cashel blue cheese on each pastry.
Carefully lift one corner and bring it to the center of the pastry. Dab a little bit of the egg wash on the center point. Repeat this with each corner until all are folded in.
Lightly brush all exposed areas of puff pastry with the egg wash and transfer to a sheet pan lined with parchment.
Bake for 20 minutes. Transfer the tarts to a serving platter and sprinkle with toasted walnuts. Finally, drizzle some of the balsamic spiked glaze over each tart and enjoy!