In a deep skillet (or large pot) over medium high heat, melt 2 tbsp butter and add onions. Cook for about 4 minutes or until onions are soft.
Add mushrooms, extra virgin olive oil and the remaining tablespoon of butter. Cook for 10 minutes or until most of the liquid from the mushrooms has evaporated. Make sure the mushrooms don’t change color (dark brown).
Add chicken stock and 2 sprigs of parsley and bring to a boil. Stir, lower heat to a simmer and cover with a lid. Cook for an hour.
Turn the heat off, remove the parsley sprigs and let cool for 10-15 minutes.
Transfer mushroom soup to a blender and blend until smooth. You can alternatively use an immersion and blend it in the deep skillet. Add a little water if you find the soup too thick.
Return mushroom soup to the deep skillet or pot, turn the heat to low and add the sherry. Stir, season with salt and pepper and serve with a little chopped parsley on top.
This mushroom soup will keep on developing in flavor so I suggest making it a day before if you want more intensity in your bowl!