For the pea puree:
Chop the onions (the white and green part), garlic, and finocchio. In a large saucepan, saute them in olive oil. Remove them from the pot, and sauté the langoustines with the shell, in the remaining olive oil. Once they get a reddish color , add the lemon and lime juice, salt and freshly ground pepper, cover with the lid, and lower the temperature to half.
In the meantime, boil the peas in salted water along with the two potatoes cut in pieces, until the potatoes melt. Keep a cup of hot water in which they have boiled.
As soon as the langoustines are ready – they don’t take more than 10 minutes - remove them from the pot and transfer them into a large bowl. Let the sauce from the langoustines' broth and the citrus to continue boiling and add the mixture of sautéed onions, garlic and finocchio. Add the chopped fennel and the dill. While your sauce is simmering, remove the shell from langoustines and bring it back to the pot, along with their juices from the bowl. Add the cardamom and half of the citrus zest and turn off the heat.
Put the peas and the potatoes in a blender or a mill and start mush them by dripping a little bit of the boiled water that you kept until you have a velvety puree.
On a platter, add the pea puree and pour the rich sauce with the langoustines. Sprinkle with the remaining zest of citrus and add a few drops of olive oil.