Heat oven to 220C/fan 200C/gas 7. In a bowl, mix together the oil and spices. Add the vegetables and mix well. Spread over a heavy baking sheet, then roast for 30 mins until tender.
Spoon into a food processor or liquidiser with half the stock and process until smooth. Pour into a pan with the remaining stock, season, then heat until barely simmering. Remove from the heat and stir in the lemon juice. Garnish with cumin seeds.