Place the marron in the freezer for 20 minutes to put to sleep, then remove the head (discard) and peel the shell from the tail, leaving the end of the tail attached. Set the yabbies aside.
Roll the brik pastry up into a log and cut into 5mm wide strips then unroll. Wrap the strips of pastry around each of the marron tails, brushing a little egg white on the end to stick together (2 marron per sheet). Set aside on a tray until required.
To prepare the garlic aioli, mix the garlic and the mayonnaise together and season to taste with sea salt, set aside.
To prepare the gremolata, mix parsley, garlic and extra-virgin olive oil in a bowl, then just before serving, add the lemon juice and season to taste with sea salt and freshly ground black pepper.
Half-fill a deep fryer or heavy based saucepan with oil and heat to 180°C (a cube of bread dropped in should brown in 15 seconds). Add the marron and cook, turning occasionally, until crisp and golden (3-4 minutes, be careful, hot oil will spit). Drain on absorbent paper and season with salt.
To serve, place a spoonful of aioli in the centre of the plate and arrange two pieces of marron on top, spoon a little gremolata onto the plate and garnish with some baby nasturtium leaves.
Note: Brik pastry is a fine, paper-thin pastry similar to filo, available from Middle Eastern grocers.