In a pot add the butter, add the onions, stir and add the rice, keep stirring till the butter has melted and absorbed by the rice.
Cover the rice with the white wine, stir and let it bubble till the wine absorbed by the rice. Add fish stock to cover the rice and bring to a boil, immediately cover with a lid, take of the stove and let it rest. Add the parmesan cheese and stir it in.

Fry the crayfish in butter with the langoustines, add salt and pepper, little bit lemon juice. Add the cognac and flambe it.
Put the risotto in a serving plate add the crayfish and the sauce on top and finish with little bit of chopped parsley.