For the mushroom cream soup, melt half of the butter in a pan over medium heat. Add the cleaned and sliced mushrooms and season with salt and pepper. Saute for about 8 minutes, until golden brown.
Remove the pan from the heat and melt the remaining butter in a saucepan. Chop the peeled onion and shallot, add to the melted butter and saute until soft, 2 to 3 minutes. Stir in the flour and saute for another 2 minutes until a thick paste forms.
Add vegetable broth, wine or sherry and stir vigorously. Set aside a quarter of the mushrooms, add the remaining mushrooms to the pot. Reduce heat and simmer, covered, for about 20 minutes. Stir occasionally during this process.
Puree the soup with a blender until smooth and stir in the remaining mushrooms, whipping cream and chopped parsley. Heat over low heat for about 5 minutes, season to taste with lemon juice and, if necessary, season the mushroom cream soup with salt and pepper.