Bring a large pot of salted water to a boil. Add the spaghetti, and cook according to package directions.
Place the mussels in a pot with some water. Cover and steam until the shells have opened and discard those which remain closed.
In a saucepan, heat the oil and butter, and saute the onion and garlic until translucent. Add tomatoes, tomato paste, broth, wine, sage, chili pepper, salt, and pepper to the onions. Let simmer for 10 minutes over moderate heat, stirring occasionally. Add mussels and simmer for an additional 5 minutes.
Remove mussels from the sauce and set aside.
Drain the pasta and toss with sauce, parsley, and grated cheese.
Divide pasta among 4 or 6 bowls depending on how large you like your servings and divide the mussels among the bowls.