Soak the dried mushrooms in lukewarm water for 20 minutes. In the meantime, peel and chop the onion.
Chop the chicken livers and chicken breast into a very small dice. Melt 3 tablespoons of butter in a pan over medium heat, add the onion and let it brown for a few minutes. Drain the mushrooms, reserving a small amount of the water in which they soaked, chop them into fairly large pieces, add them to the pan and cook for a few minutes before adding the meat.
Brown the meat for about 5 minutes, add the tomatoes and lower the heat. Salt and pepper to taste and cook for about 15 minutes, adding enough chicken stock and strained water from the mushrooms to keep the sauce from becoming too dry.
In the meantime, cook the fettuccine in a large pot of lightly salted boiling water. Drain the pasta and sauté in the pan with the sauce prepared as above.
Stir in the remaining butter and the grated Pecorino cheese and serve piping hot.
