This dressing derives its name from the wholegrain mustard used, known as ‘moutarde à Vancienne’ in Trench. It is shown here with a potato and chive salad.
Place the mustard and vinegar in a bowl with salt and pepper, to taste, and whisk together.
Sit the bowl on a tea towel to prevent it from moving while adding the oil. Gradually add the oil, pouring it in a thin steady stream, while whisking continuously. This will prevent the vinegar and oil separating.