Place onion, pumpkin and carrot on a roasting tray. Sprinkle with oil, cumin seeds, salt and pepper then put in the oven for 20 minutes. The vegetables should be soft and slightly caramelised but not mushy.
In a large saucepan place the stock, bay leaves and roasted vegetables. Bring to the boil and simmer for a further 20 minutes. When the vegetables are soft remove the bay leaves and any outer onion leaves which look fibrous then blend soup until smooth.
Adjust seasoning to taste and serve with sour cream and crusty bread.