Peel and finely chop the onion.
Wash the lettuce heart, drain, and chop it. Melt the butter in a sauté pan, add the chopped onion, and gently sauté for 2 minutes without browning. Add the chopped lettuce heart and peas, stir to combine, then pour in 2 liters of boiling salted water. Cover and simmer for 30 minutes.
Remove the heads from the langoustines and carefully shell the tails. Season with salt and pepper. Set aside.
Pass the pea mixture through a food mill or blender until smooth. Transfer to a saucepan, adjust seasoning, bring to a boil, stir in the whipping cream, and keep warm over low heat.
Heat the sunflower oil in a frying pan over high heat. Add the langoustine tails and sauté for 2-3 minutes, turning occasionally, until browned. Drain on paper towels and set aside.
Ladle the cream of peas into bowls or soup plates, top with the langoustine tails, garnish with fresh mint leaves, and serve immediately.
