Place the bread cubes in a large mixing bowl. Pour the warmed milk over the bread cubes and let it sit for about 15 minutes to soften.
In a skillet, melt the butter over medium heat. Add the finely chopped onion and sauté until soft and translucent. Remove from heat and let it cool slightly.
Add the sautéed onion, eggs, chopped parsley, salt, pepper, and ground nutmeg (if using) to the softened bread cubes. Mix well until all ingredients are evenly combined.
With wet hands, form the mixture into balls about the size of a tennis ball. You should get about 8-10 dumplings.
Bring a large pot of salted water to a boil. Reduce the heat to a gentle simmer. Carefully place the dumplings in the simmering water and cook for about 15-20 minutes, or until they float to the surface and are cooked through.
In a large pot, boil the peeled and quartered potatoes until tender, about 20 minutes. Drain and let them cool slightly.
Pass the potatoes through a potato ricer or mash them until smooth. Let the mashed potatoes cool completely.
In a mixing bowl, combine the cooled mashed potatoes, egg, flour, potato starch (or cornstarch), salt, and pepper. Mix until a smooth dough forms.
With lightly floured hands, form the potato mixture into balls about the size of a golf ball. You should get about 12-15 dumplings.
Bring a large pot of salted water to a boil. Reduce the heat to a gentle simmer. Carefully place the dumplings in the simmering water and cook for about 15 minutes, or until they float to the surface and are cooked through.
Enjoy your meal!