Spicy root & lentil casserole

INGREDIENTS

• 2 tbsp sunflower or vegetable oil
• 1 onion , chopped
• 2 garlic cloves , crushed
• 700g potatoes , peeled and cut into chunks
• 4 carrots , thickly sliced
• 2 parsnips , thickly sliced
• 2 tbsp curry paste or powder
• 1 litre/1¾ pints vegetable stock
• 100g red lentils
• a small bunch of fresh coriander , roughly chopped
• low-fat yogurt and naan bread, to serve