Melt the butter in a large saucepan over medium heat.

Add the onion, celery, and garlic to the saucepan.

Cook for about 4 minutes, or until softened.

Stir in the mushrooms.

Cook for 5-6 minutes more.

Add the vegetable or chicken stock and potatoes to the saucepan.

Bring the mixture to a boil.

Reduce the heat and simmer gently for 30 minutes.

In batches, process the soup in a blender or food processor with the lid ajar until coarsely pureed.

Sprinkle in the parsley.

Season with salt and pepper to taste.

Serve hot.

For an extra flavor, add a spoonful of prepared horseradish to each bowl of soup (optional).