In a large saucepan or high pan, heat the butter and oil over medium-high heat. Add the onion slices and sauté for 4 minutes, stirring frequently. Add the mushroom slices, reduce to medium heat and put the lid on so the mushrooms can infuse juice. Steam the mushrooms with the onions for 2 minutes, stirring occasionally.
Remove the lid from the pan and turn it up to medium-high temperature. Sauté the mushrooms and onions for about 10 minutes, stirring occasionally, until all the liquid has evaporated. Then fry again for 5 to 10 minutes until the mushrooms and onions are golden brown. Stir constantly so that nothing can burn.
Add the chicken stock and sherry and season with salt and a little pepper. Let the soup simmer for another 15 minutes. Fold in the cream and chopped parsley and heat the soup again before serving.
This soup can be prepared the day before and stored in the refrigerator. It then tastes milder and more rounded after being warmed up.