Easy mushroom soup

Name: Easy mushroom soup
Prep time: 10 min
Cook time: 15 min
Total time: 25 min
Cuisine: European, Mediterranean, French
Ingredients: 2 small/1 large onion (diced); 2 cloves garlic (finely chopped); a few fresh sage leaves (roughly chopped); 250 g chestnut mushrooms (sliced); 500 ml vegetable stock (I use Marigold Suisse Bouillon, available from most supermarkets and health food shops); 100 ml semi-skimmed milk; salt & pepper; low-calorie cooking spray