Pasta al Limone cheese curdling issue

I've attempted lemon pasta twice and both times the cheese clumps and is separated from the sauce. I'm using a nice parmesan, shredding it myself, and incorporating pasta water. I'm also using very low temperatures, and am unsure what the issue could be. I'm following this recipe which uses an emersion blender with the cheese before adding it, which I don't have. https://www.youtube.com/watch?v=ktsJZQ50Opo If anyone has any suggestions for my third attempt, I would appreciate it! submitted by /u/Hollos