Break up the cooked fish with a fork in a bowl. Season with salt and freshly ground black pepper.
Divide the fish mixture into 12 even portions, then mould into finger-shapes about 7.5cm/3in long and 2.5cm/1in thick.
Dredge the fish fingers in the flour, then dip into the beaten egg.
Dip the fish fingers into the polenta to coat completely.
Heat the oil in a frying pan over a medium heat. Add the polenta-coated fish fingers and fry for 2-3 minutes, turning regularly, until golden-brown ll over and warmed through.
For the tartare sauce, place all of the tartare sauce ingredients into a bowl and mix well.
To serve, place a spoonful of tartare sauce onto each bun, then add a little rocket.
Top each with three fish fingers and a squeeze of lemon.