Press sheets of black pasta through the chitarra and set aside. Bring 6 litre water to boil and add 2 tablespoons salt. In a 12to 36 cm sautJ pan, heat olive oil until smoking. Add onion and cook until softened, about 3 minutes. Add mussels, pepper and white wine. Cover and cook until mussels have just opened, about 2 minutes. Add tomato sauce and bring to boil. Meanwhile, drop pasta in boiling water and cook until tender, about 1 minute. Drain well and toss into pan with mussel mixture. Add mint leaves, toss pasta to coat and serve.