01 - Boil the diced potatoes in salted water until tender, about 15 minutes. Drain and let cool slightly, then mash until smooth.
02 - In a large bowl, combine the mashed potatoes, flour, egg, and salt. Mix until a soft dough forms.
03 - Divide the dough into small pieces and roll them into balls. Press a thumbprint into the center of each ball to create a small indentation.
04 - Bring a large pot of salted water to a boil. Cook the dumplings in batches until they float to the surface, about 3-4 minutes. Remove with a slotted spoon and set aside.
05 - In a skillet, melt the butter over medium heat. Add the onion and garlic, and sauté until translucent.
06 - Add the sliced mushrooms and cook until soft, about 5 minutes.
07 - Pour in the vegetable broth and bring to a simmer. Stir in the sour cream and season with salt and pepper.
08 - Serve the dumplings topped with the mushroom sauce and garnish with fresh parsley.