This is the quintessential salad dressing.
In a small mason jar, add equal amounts of red wine vinegar and extra virgin olive so that the combined liquids fill the jar about halfway. Add a heaping spoonful of dijon mustard, a generous pinch of sea salt and some fresh ground pepper. Secure the lid tightly and shake well to mix. Taste to correct the balance of ingredients. I prefer slightly more vinegar to oil. If the dressing tastes bland but is plenty acidic, add more mustard.