Shake the oil, vinegar and mustard together in a glass covered jar. Season with salt and pepper to taste, if desired. The vinaigrette is ready to use or may be refrigerated in a covered container for up to a week. Bring to room temperature before using. You can warm the jar in a bowl of warm water if you are in a rush. Shake well right before using.
We like a sharp vinaigrette with a slightly higher ratio of vinegar; the classic ratio is 3 to 1 oil to vinegar. We encourage you to use this recipe as a baseline but you can play with the ratios to your taste.