Combine vinegar and mustard in a small container; slowly add olive oil while mixing** to make an emulsion. Season to taste.
When a recipe calls for just a few ingredients, use the best quality available. My preferred champagne vinegar is "O" and when I am very fortunate, I have olive oil from Chile (thank you Marcelo and Gisela) to use in this recipe.
** A small battery-operated whisk by Bonjour (https://smile.amazon.com/BonJour-Stainless-Hand-Held-Battery-Operated-Beverage/dp/B001DJ0ZU2/ref=sr_1_sc_2?ie=UTF8&qid=1483030563&sr=8-2-spell&keywords=bon+jour+battery+operated+whisk) is perfect to quickly mix the dressing.