Cut each onion half into crescent shaped pieces about 1.5 cm thick. Toss in a roasting tin with the olive oil, balsamic vinegar and seasoning. Roast at 190 C, 170 Fan Gas 5 for 30-35 minutes, shaking the tin now and then, until tender and slightly charred.
Cook the pasta according to packet instructions until al dente. Drain and return to the saucepan. Add the onions and the rest of the ingredients. Stir and serve.