Peel the shallot and use a mandoline to cut very fine onion rings.
Heat the vinegar in a saucepan and dissolve the sugar in it.
Take the vinegar off the heat when the sugar is dissolved and pour over the shallots, making sure all shallots are covered.
Leave to pickle for at least an hour.
Marinade
Mix some vadouvan with olive oil and marinade the coquilles in it.
Coquilles
When the onions are pickled, bake the coquilles shortly in butter.
Plating
Put one leaf of baby Romaine on each plate, put a coquille on it, add some melted butter and garnish with the pickled shallots (and a cress if you like).