Heat oil in a saucepan and fry onions until soft. Set aside.
Cook pasta according to packet instructions and drain, reserving 1 cup (250ml) cooking water.
Toss hot pasta with yoghurt, cheese and reserved water and mix well.
Fry sage leaves in butter until crisp, then allow butter to brown slightly.
Spoon fried onions over pasta and top with sage and browned butter.
Sprinkle with extra cheese and serve