Baked dhal with tamarind-glazed sweet potato

Name: Baked dhal with tamarind-glazed sweet potato
Cuisine: Asian, Indian
Ingredients: 250g split red lentils; 2 tbsp coconut or groundnut oil; 1 onion, peeled and finely chopped; A large thumb-sized piece of ginger, peeled and finely chopped; 2 cloves of garlic, peeled and finely chopped; 1 tsp cumin seeds; 10 cardamom pods; 1 tsp ground turmeric; A pinch of dried chilli flakes; 1 x 400g tin chopped tomatoes; 750g hot vegetable stock; 1 x 400ml tin coconut milk; 750g sweet potatoes; 3 tbsp tamarind paste; 2 unwaxed limes, halved