Chop one large onion. Melt two ounces of butter (or heat 2 ounces oilve oil, or a combination) in a large pan. Sweat the onions until transparent -- do not let them brown. If you don't mind garlic, add one clove, crushed.
Add 1 1/2pounds of mushrooms, sliced. Kind of stir those around for a bit over medium heat, until they are soft.
Add about 1 litre of stock -- vegetable or chicken usually. (I use good stock cubes.)
Season with herbs (oregano, parsley, etc.), fresh if possible, and a few good grinds of coarse black pepper.
Simmer for 20 minutes or so.
Let cool. Use a blender to puree the soup -- leaving about 1 cup unblended if you like a bit of chunkiness in the finished product.
To serve, reheat to just below boiling. Ladle into bowls. Decorate with a swirl of fresh cream (if you like that sort of thing), a slice of raw mushroom, and a sprinkling of chopped herbs.