Curried carrot soup

Name: Curried carrot soup
Ingredients: 150g/5½oz unsalted butter; 150g/5½oz onions, chopped (about 2 onions); 2 garlic cloves, crushed; 500g/1lb 2oz carrots, peeled and chopped into small pieces; 1 tsp cumin seeds; ½ tsp Madras curry powder; 300ml/½ pint chicken stock (or vegetable stock); 1 bouquet garni; 1 tbsp chopped fresh coriander, to serve; salt