Melt half the butter in a large saucepan or deep-sided sauté pan and gently fry the shallots until softened. Add the chilli, paprika and garlic and fry gently for 30 seconds, without browning the garlic.
Sprinkle in the rice and fry gently for 1 minute, stirring. Add the wine and let it bubble until almost evaporated.
Add the thyme and a ladleful of the stock and cook, stirring, until the rice has almost absorbed the stock. Continue cooking, adding the stock a ladleful at a time, and letting the rice absorb most of the stock before adding more. Once the rice is tender but retaining a little bite, the risotto is ready – this will take about 25 minutes. You may not need all the stock.
Stir in the tarragon, crayfish and remaining butter and heat through gently for 1 minute. Add a little extra salt if necessary and serve immediately, garnished with Parmesan and with a watercress salad, if you like.