Okra and potato masala

Name: Okra and potato masala
Ingredients: 1 teaspoon black mustard seeds; 1 teaspoon cumin seeds; 1 medium onion (diced); 2 cm chunk fresh ginger (peeled and grated); 2 fat cloves of garlic (peeled and grated); 1 medium red or green chilli (deseeded and finely chopped); 2 large tomatoes (chopped); 1 teaspoon red chilli powder (kashmiri if possible); 1/4 teaspoon ground turmeric; 1 teaspoon ground coriander; 2 medium potatoes (scrubbed and diced); 1 tbsp tomato purée; 250 grams okra (washed, trimmed and cut into 3cm chunks); salt and pepper; 2 tbsp fresh coriander leaves (roughly chopped); ½ teaspoon garum masala