I don't think it was sherry vinegar, as the dressing was light in color, a pale yellow, and knowing Chef Beran, he probably started with fresh pears. I'll have do google to see if similar dressings were on menus of the Alinea Group restaurants where he used to work. My guess is that it is a classic french vinaigrette recipe, the one that is made with dijon mustard, with the addition of some poached pear puree and possibly a substitution of pear vinegar for the wine vinegar.