Sweet potato, spinach & lentil dahl

INGREDIENTS

• 100g red lentils
• 450ml vegetable stock
• 1 small onion , grated or finely chopped
• 2 tomatoes , chopped
• ½ tsp turmeric
• 1 tsp garam masala
• 1 red chilli , finely chopped
• 1 large sweet potato , cut into small pieces
• 2 handfuls young leaf spinach , shredded
• 2 naan bread , warmed, to serve