Cacio e pepe, literally 'cheese and pepper', is a very typical Roman dish and tonnarelli is a kind of square spaghetti made with egg pasta popular in Le Marche, Lazio, Abruzzo, Molise and Puglia. At its simplest, literally all you do is boil the pasta in well salted water, drain it (but not very well) and pour it into a heated bowl. Then add a generous amount of pecorino cheese and lots of freshly ground pepper, and mix well. The hot water that clings to the pasta should meld with the cheese to make a kind of creamy sauce. If the dish is too dry, you can add a bit more of the pasta water. If, on the other hand, you can still see water at the bottom of the bowl, add more cheese. Then top with more pecorino and another healthy grinding of pepper. That's it!