For the mackerel tartare, mix all of the mackerel tartare ingredients, apart from the whole mackerel fillet, together in a bowl until well combined. Chill in the fridge until needed.
Preheat the grill to a medium setting. Place the whole mackerel fillet on a baking tray and grill for 2-3 minutes, or until browned on top and cooked through. Set aside.
For the shallot rings, heat the rice wine vinegar and sugar in a non-reactive saucepan over a medium heat. When the mixture is just coming up to the boil, add the shallot rings, celery stalk, radishes and salt. Set aside to macerate for 5 minutes. Stir in the celery leaves just before serving.
For the croûtons, heat the oil in a heavy-based frying pan over a medium heat until a breadcrumb sizzles and turns golden-brown when dropped into it. (Caution: Hot oil can be dangerous. Do not leave unattended.) Add the baguette slices to the hot oil, in batches if necessary, and fry for 2-3 minutes on each side, or until golden-brown on both sides and crisp. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper.
To serve, divide the croûtons equally among four serving plates. Top each croûton with a spoonful of the mackerel tartare. Spoon the shallot rings and macerating liquid alongside. Garnish with the coriander cress and a small piece of grilled mackerel.