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.