AH... I missed that in peanuts recipe, just assumed it was there...

we always use cubed or thin sliced fresh mozzarella...
or a cheese shop nearby also makes grape sized fresh mozzarella... and we just halve them...
our local Stop & Shop also carries the grape sized fresh mozzarella... (not quite as good or creamy as the local cheese shop, but half the price!)...
you need to cut them in half to get by the smooth outside, and expose the porus insides, in order for the liquids & flavors to penetrate...
It's an orgasm for your taste buds!

I'll have to try substituting sherry for the balsamic vinegar sometime! It would give it certain sweetness...

Peanut, do you use a dry, medium dry or sweet sherry?
I usually use a medium dry sherry for most recipes...