Heat 2 tbsp of the oil in a pan over a medium heat and fry the onion gently for 10-12 mins. Add the garlic and chilli flakes and cook for 1 min. Tip in the tomatoes and sugar and season to taste. Simmer uncovered for 20 mins or until thickened, then stir through the mascarpone.
Heat 1 tbsp of oil in a non-stick frying pan. Season the chicken and fry for 5-7 mins or until the chicken is cooked through.
Heat the oven to 220C/200C fan/gas 7. Cook the penne following pack instructions. Drain and toss with the remaining oil. Tip the pasta into a medium sized ovenproof dish. Stir in the chicken and pour over the sauce. Top with the cheddar, mozzarella and parsley. Bake for 20 mins or until golden brown and bubbling.