Put a large pan of 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 till golden.
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. Cook for 5 mins till juicy.
Peel and thinly slice the garlic. Roughly chop the olives. Add the garlic and olives. Cook and stir for 1 min. Pour in the chopped tomatoes. Stir together. Cover and simmer for 20 mins.
When the sauce has cooked for 10 mins, add the spaghetti 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. Drizzle over ½ tbsp oil and season with salt and pepper.
Taste the sauce. Add more salt and pepper if you think it needs it. Stir through most of the shredded sage leaves.
Divide the spaghetti between two warm bowls. Spoon over the sauce. Sprinkle over the remaining shredded sage and serve with the Parmesan cheese for grating.