agree with PPs that it wont work well. Not sure the flavor would be right either.

No - it doesn't melt well enough.
If you are making a trip to the store, might I recommend Taleggio? My new favorite cheese to cook with - it melts beautifully like a young cheese, and has a lot of flavor, like a mature one.

I think Manchego is too hard - that sounds like they want something that will melt like Fontina or Guyere. I think you could sub it for the Parmesan though.

recipe calls for a "semi hard" cheese but all I have is manchego. Do you think it'll work? Don't want to go to the store....

http://www.nytimes.com/2006/10/04/dining/043mrex.html