To make the soup, melt 2 tablespoons of butter over a medium heat in a large saucepan. Add the onions and sauté for 5 minutes, stirring frequently. Add 200g of the sliced mushrooms and sauté until they have released their liquid and started to brown. Stir in the garlic and a tablespoon of chopped parsley, cooking for another minute. Stir in the flour until it has absorbed all the buttery juices and cook for 1-2 minutes until lightly browned. Gradually add the broth, stirring with a wooden spoon to make sure there are no lumps. Pour in the milk or cream, season and cover. Leave it all to simmer over low heat for about 10 minutes, stirring occasionally. In the meantime, follow the instructions on the pack to cook the tortelloni. Melt the remaining butter in a frying pan and sauté the remaining mushrooms until golden and softened. Purée the soup in a blender, adding more stock or water if it is too thick. Pour back into a saucepan, add the tortelloni and sautéed mushrooms and bring back to the boil. When it's ready, divide into bowls and sprinkle with the remaining parsley and a twist of freshly ground black pepper.