Preheat oven to 210℃/410℉.
Cut each puff pastry sheet into six rectangles. Place on baking trays lined with baking paper.
Score a second rectangle inside the pastry rectangles to allow for an edge to the tartlet. Ensure you don't cut through the pastry.
Cook in oven for 5-10 minutes, until pastry starts to rise a little.
Remove from oven and smear a little bit of jam inside the scored second rectangle. Place blueberries into the jam. Sprinkle with a little caster sugar.
Put tartlets back into the oven for a further 20 minutes or until lightly browned.
Remove from oven. Sprinkle with crushed nuts and additional sugar.
For lemon icing, combine all ingredients until smooth. Drizzle over tartlets once they have cooled down. Sprinkle with icing sugar for effect.