This is a traditional French salad dressing. Make a big batch and keep it in the fridge, it will keep for at least a week, but bring to room temperature before using. Makes about ½ cup
Measure the mustard and vinegar into a small bowl and whisk until blended. Very gradually whisk in the olive oil until the mixture thickens. Then whisk in the oil in a thin steady stream. Season to taste with salt and pepper. Mix in any optional ingredients. (Can be stored in the refrigerator. Bring to room temperature before using.) Use to dress any salad.