Put a large pan of salted water on to boil. Heat 1 tbsp oil in a large frying pan over a medium heat. Tip the diced chicken into the pan and fry for 5 mins, turning once or twice, till golden all over.
While the chicken fries, peel and finely chop the onion. Halve the pepper and scoop out the seeds and white bits. Chop the pepper into small chunks. Slice the mushrooms. Add the veg to the pan. Season with salt and pepper. Cook for 5 mins till juicy, stirring now and then.
While the veg fry, peel and thinly slice the garlic. Roughly chop the olives. Add the garlic and olives to the pan. Cook and stir for 1 min. Pour in the chopped tomatoes, then add 200ml cold water. Stir together, then cover and simmer for 20 mins. Stir every so often.
When the sauce has cooked for 10 mins, add the chifferini to the pan of boiling water. Simmer for 8 mins till al dente (soft with a slight bite).
While everything cooks, finely shred the sage leaves and set aside. Drain the pasta and tip back into the pan.
Taste the sauce. Add more salt and pepper if you think it needs it. Stir through most of the shredded sage leaves.
Divide the chifferini between 2 warm bowls. Spoon over the sauce. Sprinkle over the remaining shredded sage and serve with the parmesan cheese for grating.