Spiced root vegetable soup

INGREDIENTS

• 2 tbsp olive oil
• 2 onions , finely chopped
• 2 sweet potatoes , chopped
• 2 carrots , chopped
• 2 parsnips , chopped
• 1 red chilli , roughly chopped
• 1 tbsp ground cumin
• 75g dried green lentils
• 1.3l vegetable stock
• 425ml milk
• 100g Greek yogurt
• 1 tbsp coriander leaves, chopped