For the bisque, heat the oil and butter in a wide frying pan and sauté the shallots and fennel for 2-3 minutes.
Add the flour and tomato purée and cook, stirring, for a further minute.
Add the crayfish shells and brandy and cook for another minute.
Add the fish stock, bay leaves and parsley and bring to a simmer. Cook for 20-25 minutes, or until the liquid has reduced by a third. Remove from the heat and allow to cool slightly.
Pour the mixture into a food processor and blend to a fine purée.
Pass the purée through a sieve into a clean saucepan and season, to taste, with salt and freshly ground black pepper.
For the risotto, place the fish stock into a saucepan and bring to a gentle simmer.
Meanwhile, heat the butter in a saucepan, add the onion and leeks and cook gently for a few minutes, until softened but not coloured.
Add the rice and cook for a minute, stirring to coat the rice in the butter.
Add the wine and simmer until reduced by half.
Add the warm stock a ladleful at a time, stirring well between each addition until nearly all the stock is absorbed and the rice is just tender.
Add two thirds of the crayfish tails and 200ml/7fl oz of the bisque and stir to combine. Stir in the mascarpone, parmesan and salt and freshly ground black pepper, to taste.
To serve, spoon the risotto into the centre of a large soup plate. Place the remaining crayfish tails around the edge of the risotto.
Using a hand blender, blend the remaining bisque until frothy. Spoon the foam over the crayfish tails.