We loved this recipe!! I had fresh mozzarella on hand so that's what I used and just cut it into very small cubes before stuffing the burgers. It worked great. I just made a double batch of the sauce to freeze. I couldn't resist eating some of it right away so I drizzled some flat bread with olive oil, sprinkled with a little grated pecorino romano and fresh mozzarella and broiled it. Then poured the sauce over it for a very tasty lunch!! Will definitely be making this recipe again and will be stocking the freezer with the sauce!