Heat a little oil in a large frying pan over a medium heat. Add the onion and garlic and cook for about for 5 minutes until just softened. Turn up the heat, add a little more oil and then add the chicken. Cook for about 5 minutes, stirring frequently, until browned all over. Stir in the chopped tomatoes, water, tomato purée, crème fraiche, sugar and basil. Season with salt and freshly ground black pepper and simmer gently, uncovered, for 20 minutes. Meanwhile, cook the pasta according to the packet instructions.
Preheat the grill to high.
Drain the pasta and stir into the chicken and tomato sauce. Transfer to a shallow heatproof dish and sprinkle over the chopped mozzarella. Place under the grill until the cheese is golden and bubbling – this should take about 5 minutes.
Garnish with basil leaves and serve with a baby kale salad.