This is not a copy of a recipe, but my memory of some instructions given to an interviewer of Raymond Blanc in a Sunday magazine. Its importance lies not in the ingredients, which are standard, but the method.
Put a teaspoon of Dijon mustard into a mug. Add some sea salt and freshly ground black pepper. Then add olive oil, as though you were making mayonnaise, slowly at first and then a little more quickly, stirring vigorously, until you have a mixture so thick you could turn the mug upside down. Then add about a teaspoon of vinegar.
The advantage of doing it this way is that you have an uncurdled vinaigrette which coats every leaf beautifully.