Toast cumin, coriander, and cardamom in a frying pan over medium heat for one minute.
Grind the toasted spices into a fine powder using a pestle and mortar.
Heat vegetable oil in a large pan and fry sliced onions for 20 minutes until golden-brown.
Add garlic, ginger, ground spices, curry powder, chilli powder, and cinnamon to the onions.
Cook the spice mixture for an additional five minutes.
Stir in tomatoes, coconut milk, and 450g of sweet potatoes.
Simmer for 20 minutes or until the sweet potatoes are tender.
Add 200g of peas five minutes before the cooking time ends.
Serve the curry with basmati rice and mango chutney.