Sauté shallots and fennel in oil and butter for 3 minutes.
Stir in flour and tomato purée and cook for 1 minute.
Add crayfish shells and brandy and cook for another minute.
Add 1 litre fish stock, bay leaves, and parsley and simmer for 25 minutes.
Blend the mixture in a food processor to a fine purée.
Sieve the purée into a saucepan and season with salt and pepper.
Bring another 1 litre of fish stock for the risotto to a gentle simmer.
Sauté onion and leeks in butter until softened.
Add rice and cook for 1 minute while stirring to coat.
Add wine and simmer until liquid is reduced by half.
Gradually add warm stock by the ladleful until rice is tender.
Stir in two thirds of crayfish tails, 200ml bisque, mascarpone, and parmesan.
Spoon risotto into plates and garnish with remaining crayfish tails.
Aerated the remaining bisque with a hand blender and foam over the tails.