Melt the butter in a heavy-based frying pan. Add the rice and cook for 2-3 minutes on a low heat. Once the rice starts to become translucent around the edges, add the wine and cook for a further minute.
Add a ladleful of hot stock. When all the liquid has been absorbed add another ladle and continue doing so until the rice is tender.
Stir in the sun-dried tomatoes, chilli, spring onion, lime zest and season to taste with salt and freshly ground black pepper. Remove from the heat and set aside. Once the rice is cooled, fold in the crab and tarragon.
Divide the rice mixture into six and roll into balls. Flatten each ball slightly to make a patty shape.
Sprinkle the flour and breadcrumbs onto separate plates and beat the egg in a bowl. Dredge the balls in flour, dip into the egg, then roll in the breadcrumbs.
Half-fill a deep, heavy-bottomed saucepan or deep fat fryer with vegetable oil and heat until a small cube of bread sizzles and turns golden-brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Fry the risotto cakes for 3-4 minutes, or until golden-brown then remove from the pan with a slotted spoon and set aside to drain onto kitchen paper.
For the broad bean purée, blend the broad beans, thyme, garlic and lemon in a blender and until smooth.
With the motor still running, pour in the olive oil and blend until smooth.
To serve, spoon some broad bean purée on serving plates and top with the risotto cakes.