Cook the pasta in boiling salted water according to the instructions on the packet. Peel and finely chop the onion. Heat oil in a pot and sauté onion. Add the chopped tomatoes, season with salt, pepper and sugar, bring to the boil.
Wash the chillies and cut into rings. Wash basil, dab dry, pluck leaves from the stems and cut into strips. Add chilli and basil to the sauce. Drain and fold in the pasta.
Arrange on plates and garnish with sage, chillies and pecorino.