Dahl with Sweet Potato Spirals

Dahl

• 1 onion, finely chopped
• 2 tbsp butter
• 3 garlic cloves, grated (see note)
• 1 tbsp fresh ginger, grated
• 1 ½ tsp curry powder
• 1 ½ tsp ground garam masala
• 1 tsp ground turmeric
• 4 cups (1 litre) water
• 1 ½ cups (285 g) dried red lentils, rinsed and drained
• ½ tsp sugar

Toppings

• 5 cups (340 g) sweet potatoes, cut into spirals (see note)
• 2 tbsp (30 ml) vegetable oil
• ½ tsp ground cumin
• Cilantro leaves, to taste
• Plain yogurt, to taste
• 4 naan breads, warmed
• 1 lime, cut into wedges