Sweet potato curry

Name: Sweet potato curry
Prep time: 15 min
Cook time: 1 h
Total time: 1 h 15 min
Cuisine: Asian, Indian
Ingredients: 1 tsp black mustard seeds; 2 tsp cumin seeds; ½ tsp hot chili flakes; 10 dried curry leaves; 1½ tsp ground coriander; ½ tsp turmeric; ½ tsp cinnamon; ¼ tsp ground cardamom; a good grind of black pepper; 1 tsp garam masala, divided; 2 tbsp vegetable oil (I use mild olive oil); 1 onion, finely chopped; 4 garlic cloves, finely chopped; a thumb piece of ginger (2 tbsp very finely diced); approx. 1 tsp fine sea salt, adjust to taste; 400 g / 14 oz can of peeled plum tomatoes; 500 g / 17.5 oz sweet potatoes (2 medium); 400 g / 14 oz can full fat coconut milk; 400 g / 14 oz can chickpeas (optional); handful of cashews (optional); juice of ½ lime; handful of coriander/cilantro, chopped