Place the cold mashed potatoes in a large mixing bowl. Add the egg, salt, and gradually incorporate the flour, mixing until you have a smooth dough.

Divide the dough into small pieces. Roll each piece into a ball and flatten it slightly.

Bring a large pot of salted water to a boil, then gently add the dumplings. They are ready when they float to the surface, usually 3-5 minutes.

Remove them with a slotted spoon and set aside.

In a skillet, heat the olive oil over medium heat. Add the diced bacon and cook until crispy.

Stir in the minced garlic and chopped onion, allowing them to soften and become fragrant.

Add the chopped mushrooms and sauté until everything is golden and delicious.

Toss in the dumplings and gently mix until they are coated in the savory mixture. Finish with a pat of butter.