Add the onion and white wine vinegar. Whisk. Next, add the eggs very slowly and whisking vigorously to keep the eggs from scrambling. Next, whisk in the half and half, lemon zest, lemon juice, fresh tarragon, fresh parsley, salt, pepper, dry mustard, and cayenne pepper. Set the heat power setting on your microwave down to 50%, and microwave the mixture in 15 second intervals, whisking after each interval, until the sauce has slightly thickened. Cover the sauce with plastic wrap and place the sauce in the fridge for at least an hour. It will continue to thicken as it chills.