Baked dhal with tamarind-glazed sweet potato

Ingredients

• 250g split red lentils
• 2 tablespoons 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 teaspoon cumin seeds
• 10 cardamom pods
• 1 teaspoon ground turmeric
• a pinch of dried chilli flakes
• 1 x 400g tin chopped tomatoes
• 750ml hot vegetable stock
• 1 x 400ml tin coconut milk
• 750g sweet potatoes
• 3 tablespoons tamarind paste
• 2 unwaxed limes, halved