In a large frying pan, saute the onion in olive oil until soft. Meanwhile, cook the pasta in plenty of salted water until al dente, then lift into the onion pan with any residual water clinging to it. Pull the frying pan from the heat, add the pecorino, and mix well until creamy – adding more pasta cooking water if necessary. Add the torn basil and flaked fish, toss again, serving with more grated pecorino.