The simple addition of tarragon takes mayonnaise to a new level. Use it as you would regular mayonnaise, though we especially like it on this steak sandwich.
Game plan: Have all the ingredients at room temperature before you start; the mayo will come together better. Also, because the egg doesn’t get cooked, use very fresh or pasteurized eggs. Once made, the mayonnaise should be stored in an airtight container and can be kept in the refrigerator for up to 4 days.