Boil a large quantity of water with the sprig of thyme, coarse gray salt and peppercorns. Immerse the langoustines in it, let them come back to the boil and cook for 2 minutes. Remove them with a slotted spoon and set them aside.
Slice the green beans, cook them in salted water and uncovered for about fifteen minutes, remove them with a slotted spoon and plunge them into a bowl containing iced water, drain then very carefully.
Peel the carrots, cut them into thin julienne strips, blanch them for 1 minute in salted boiling water, then cool them and drain them.
Wash lettuces and radishes. Drain the lettuces and cut the radishes into rings.
Shell the langoustine tails, without breaking them.
Peel and very finely chop the shallot. Prepare the sauce by mixing the vinegar with the mustard, shallot and oil, salt and ground pepper.
On a serving platter, arrange the lettuce, green beans, radishes and shelled langoustine tails. Sprinkle at the last moment with the sauce, mix delicately and serve.
