Heat the oil in a large, lidded non-stick saucepan. Add half the chopped chilli to the pan along with the onion, ginger, chopped coriander and balti paste. Cook over a medium heat, stirring, for 4-5 minutes until the onion is soft and starting to turn golden.
Add the chicken chunks and stir-fry until browned, then add the chopped tomatoes, potatoes and 100ml water. Season with plenty of freshly ground black pepper and simmer, uncovered, for 10 minutes.
Add the frozen spinach cubes to the pan, then cover and heat through for 5 minutes, stirring occasionally. Simmer uncovered for a further 5 minutes until the chicken is cooked through with no pink colour remaining and the sauce has thickened.
Divide between 4 bowls and garnish with the reserved coriander sprigs and remaining chilli. Serve with a dollop of the yogurt and the halved naan bread on the side.