In a small bowl, mix together the chopped palm, celery seeds, leaves, seaweed, salt and tapioca starch. Set aside for 30 minutes.
Using a food processor, blitz the seeded bread to make a rough crumb. You can also use a hand grater to do this, on the largest tooth. Set aside on a small plate. On another plate, add the flour, salt and pepper, and mix well. Put the aquafaba into a shallow bowl.
Using your hands, take a small handful of the chopped palm mixture and shape into a small sausage or goujon shape. Dip into the flour and then into the aquafaba. Then roll in the breadcrumbs and lay on greaseproof paper ready for frying. Repeat with the remaining palm mixture to make five or six fingers. Cover the tray with clingfilm and place in the fridge to set for an hour or so.
Put the olive oil into a large frying pan over a medium-high heat. Drop a breadcrumb in the oil and if it sizzles without burning the oil is ready. Gently lay half of the fingers into the hot oil and fry gently for 8-10 minutes, turning once. Once crispy and golden, drain on paper towels to remove any excess oil. You can keep the fingers warm and crispy in a low oven while you complete the next batch.
Butter the bread if you like, and slather one side of each slice generously with tartare sauce, lay half of the ‘fish’ fingers on top of two slices of bread and then some rocket leaves. Top each with another thick slice of buttered bread and serve.