Bring a large pot of water to the boil and season heavily with salt. Add the langoustines, bring back to the boil and boil for one minute.
Remove the langoustines and allow to cool. When they are cool enough to handle, peel them.
To make the mayonnaise, whisk a splash of water with the vinegar, mustard, salt and freshly ground black pepper until all dissolved. Add the egg yolks and whisk in. Add the vegetable oil, drop by drop, then the olive oil. Whisk slowly until emulsified.
Cut each langoustine into two or three, then combine them with 4 tbsp of the mayonnaise, chopped basil and a little salt and freshly ground black pepper. Toss gently together, and set aside.
For the avocado layer, mix the avocado with the shallot, chives and lemon juice and season with salt and freshly ground black pepper. Set aside.
To make the vinaigrette, whisk the sherry vinegar with the Dijon mustard, sugar and a pinch each of salt and freshly ground black pepper until dissolved. Stir in the oil. Reserve two tbsp of the vinaigrette, then toss the tomatoes and shallot in the rest. Taste for seasoning.
For the homemade chips, heat the oil in a deep, heavy-bottomed saucepan. It is ready when a breadcrumb that is dropped in will sizzle gently in it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Fry the potato strips in the oil until crisp and golden. Drain on kitchen towels and season with salt and freshly ground black pepper.
To assemble, place a 4-5cm/1½-2in chef's ring in the centre of each plate. Put about two tablespoons of the avocado mixture in each ring and push down well. Top with the langoustine mayonnaise, again pushing down well. Surround each ring with a few tablespoons of the cherry tomato vinaigrette and set three quails' egg quarters on top of the tomatoes. Remove the rings.
Toss the salad leaves with the reserved vinaigrette, and carefully set a few leaves on the langoustines. Top each with a potato chip and serve.
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