Preheat the oven to 200°C/fan 180°C/gas mark 6. Cut the ciabatta into rough pieces and place in a food processor. Blitz until you have course textured breadcrumbs. Transfer to a bowl and add the flat-leaf parsley, harissa and lemon zest. Stir well so all the crumbs are evenly coated with the harissa paste.
Place the flour and breadcrumbs on two separate plates, and the egg in a shallow bowl. Dip each piece of salmon first in the flour, then in the egg, and finally the breadcrumbs, turning continuously until the whole finger is well coated. Transfer to a lined baking tray and repeat with the rest of the fillets. Chill the fingers for 15-20 minutes to firm up, then bake for 12-15 minutes, until crispy.
Meanwhile, mix together the mayo and lemon juice and season with black pepper. Serve the fish fingers with the mayo on the side.