Put the yolk or egg, mustard, salt, pepper and lemon juice or vinegar in the container of a food processor and turn the machine on. While it’s running, add the oil in a slow, steady stream. (Your food processor should have a teeny hole in the food pusher insert in the top). When an emulsion forms you can add it a little faster. (Again, the little hole makes this unnecessary.) Taste and adjust seasoning if necessary. Add the fresh tarragon (if desired) and pulse until chopped.