Masala chicken pie

Name: Masala chicken pie
Ingredients: 2 tbsp vegetable oil; 4 skinless chicken breasts; 2 onions chopped; finger-length piece ginger grated; 3 garlic cloves crushed; 2 tbsp medium curry powder; 2 tsp ground coriander; 2 tsp ground cumin; 2 tsp black or brown mustard seed; 2 tsp white or red wine vinegar; 2 tsp sugar (white or brown); 2 x 400g cans chopped tomatoes; 150ml light coconut milk (buy a 400ml can- you'll need more for the topping); 1 large red pepper deseeded and cut into large chunks; 1 large green pepper deseeded and cut into large chunks; ½ a small bunch coriander leaves roughly chopped, stalks reserved (see below); 1 ½kg floury potato cut into very large chunks; 150ml light coconut milk; 1 tsp turmeric; juice 1 lemon; 1 bunch spring onions finely chopped; stalks from ½ a small bunch coriander finely chopped; 1 tsp kalonji seeds (also known as nigella or onion seeds ); naan bread to serve (optional)