Melt 2 tbsps butter in a frying pan. Add the onion and cook gently for 5 minutes. Add the pumpkin and toss well, then add the stock, sugar, nutmeg and sea salt.
Cook gently for 20 minutes or until tender. It should still be slightly soupy.
Add the pasta to a large pan of boiling salted water and cook until al dente ( tender but firm to the bite). Meanwhile, heat the olive oil in a small pan and gently fry the whole sage leaves until crisp.
Drain the pasta and toss lightly with the pumpkin, adding the remaining 1 tbsp butter, the chopped sage, parmesan and lots of black pepper.
Divide between warm pasta bowls and scatter with the crisp sage leaves. Serve with extra parmesan at the table.