Heat a medium pan. Shred the Parmigiano-Reggiano (a good handful or two) and add this and a small splash of milk to the pan. Begin to mix and add a little more milk as it begins to milk. Add a dollop of sour cream, mixing and not letting anything clump or stick. If you do not have any more cheese to spare, but need the mixture to be a bit thicker, add a little corn starch or flour. Whisk this in, not letting it clump. Continue to add a dollop of cream, milk, or Parmigiano-Reggiano until the mixture is creamy and whisked through. As long as it's not to thin, it's fine to make to your liking. Add a few grinds of salt, pepper, and more nutmeg. Set aside on very low heat, and stir every so often so that it does not settle.