Could I use a small block of fresh yeast for this? That is the yeast I have - but I'm not sure how to use in this recipe.

If you are in Italy maybe you should ask someone there. When I go and bake at a friends in France the packaged yeast is different than ours in the US- its finer. I would guess that the yeast is a similar product in Italy and its not going to make or break it, kwim? Jim Lahey uses much less yeast and longer rises, right? Try one batch with a third of the yeast you have.
Wish I was there to try it!

If you are in Italy maybe you should ask someone there. When I go and bake at a friends in France the packaged yeast is different than ours in the US- its finer. I would guess that the yeast is a similar product in Italy and its not going to make or break it, kwim? Jim Lahey uses much less yeast and longer rises, right? Try one batch with a third of the yeast you have.
Wish I was there to try it!

It depends on the size of the block. They used to be sold two ways. If its a 2-3 oz block - it sorta looks like 3 tablespoons of butter? use a third would be like using one packet from a 3 strip of dried yeast. If its smaller than 2 oz. - looks like 1 tablespoon of butter, then it equals one packet of dried.
I have not seen it available for years and used to buy a pound of fresh yeast from a bakery or pizzaria. While not exactly the same I would divide it into 16 equal cubes, wrap them each in plastic wrap and put them in a zipper bag in the freezer. They keep very well frozen but only several days in the fridge. I had very good results with this method and baked great bread with it. Now you have me wanting to search the web and see if I can still buy it in bulk. It must be available somewhere.

The fresh yeast should work fine, but maybe a bit faster than the active dry yeast.
So two tips -
* Don't use too much fresh yeast
* Check the dough more often than Lahey recipe suggests in case you need to punch it down intermittently.
Last, have a look at this article on converting recipes of active dry yeast to fresh & vice versa. Or similar ones on the web.
http://makebread.com.au/fresh-yeast-conversion/