• Combine mustard and vinegar.
• Slowly pour in oil and wisk till smooth and creamy.
• Shake before using.

Variations (not in Nora Ephron's book):

• It works fine just to put all ingredients in a jar and shake.
• Make a quadruple recipe and store in refrigerator. Keeps well.
• Use less mustard. (I usually use less than 1 tablespoon.)
• Add salt.
• Add garlic granules.
• Add herbs, etc. etc. in addition to or instead of mustard.