Season the kabeljou fillet with salt and pepper and grill skin side down first – the flesh should change from translucent to white. Turn the fish and grill flesh side down for a few minutes until cooked. Discard the skin and flake the pieces while hot. Squeeze some lemon over the fish.
On a mandolin, thinly slice the black radish in discs – you need 24 discs (3 per serving). As you slic