Put the egg, garlic, lemon juice, parsley and green onions in a food processor and purée for 15 seconds.

With the processor running, slowly pour in the oil in a slow, steady stream, until completely incorporated and mixture is smooth and thick.

Add the cayenne, mustard and salt and pulse once or twice to blend.

Cover and let sit for 1 hour in the refrigerator before using.

Best if used within 24 hours.