Bombay potato & spinach pies

Name: Bombay potato & spinach pies
Prep time: 40 min
Cook time: 35 min
Total time: 1 h 15 min
Cuisine: Indian
Ingredients: 1¼kg large waxy potato such as Charlotte, halved; 85g butter; 2 onions chopped; 1 tbsp cumin seed; 1 tbsp black mustard seed; 2 tbsp finely chopped ginger; 2 red chillies halved, deseeded and sliced; 3 tbsp korma paste; 400g bag fresh spinach; 4 tomatoes chopped; small bunch coriander chopped; 270g pack filo pastry (6 large sheets); 50g melted butter; 1 tsp black mustard seed