Peel and chop the onion.
Wash the lettuce heart, drain it and chop it. Put the butter in a sauté pan and add the minced onion, let it come back for 2 minutes without colouring, add the heart of lettuce and the peas, mix and pour into 2 liters of boiling salted water, cover and cook for 30 minutes.
Remove the head of the langoustines and delicately shell the tails, salt and pepper them. Book.
Pass the peas through a vegetable mill or blender, pour into a casserole dish, check the seasoning and bring to a boil, add the whipping cream, mix and set aside over low heat.
Heat the oil in a frying pan, throw in the langoustine tails, let them brown for 2 or 3 minutes over high heat, turning them over, pat them dry on absorbent paper. Book.
Pour the cream of peas into bowls or soup plates, distribute the langoustines, garnish with a few mint leaves and serve immediately.
