How to Make It
In a medium heavy pot, heat 2 tablespoons of the butter over medium high heat until hot. Stir in the mushrooms and cook, stirring occasionally, until browned, about 6 minutes. Transfer the mushrooms to a plate. Peel the potatoes and cut into 1-inch pieces.
Add the remaining 1 tablespoon of butter to the pot and stir in the onion. Cook, stirring occasionally, until golden, about 6 minutes. Stir in the potatoes, stock, and half & half and bring to a boil, then cook until the potatoes are tender, about 15 minutes.
In a blender, puree the soup in batches until smooth. Season the soup to taste with salt and pepper and serve topped with the reserved mushrooms and parsley leaves.