01 - Preheat the oven to 350°F. Generously grease a 9x13-inch baking dish with a small amount of butter.
02 - Place the potatoes in a large pot and cover with cold water. Add 1 teaspoon of salt and bring to a boil. Simmer for 18 to 20 minutes until just tender but not falling apart. Drain thoroughly, let cool, then slice into ¼-inch rounds.
03 - Place eggs in a saucepan and cover with cold water. Bring to a boil, then simmer for 10 minutes. Drain, cool under running cold water, peel, and slice into rounds.
04 - Melt 1 tablespoon butter in a skillet over medium heat. Add the chopped onion and sauté gently until translucent, about 5 minutes. Remove from heat and set aside.
05 - Arrange one-third of the potato slices evenly across the bottom of the prepared baking dish. Season lightly with salt and pepper.
06 - Distribute half of the egg slices over the potatoes, followed by half of the sausage slices and half of the sautéed onions. Spread one-third of the sour cream gently over this layer.
07 - Repeat with another layer of potatoes, remaining eggs, remaining sausage, remaining onions, and another third of the sour cream.
08 - Complete with a final layer of potatoes. Season with any remaining salt and pepper, then spread the remaining sour cream evenly over the top. Dot with small pieces of the remaining butter. Sprinkle with breadcrumbs if using for extra crunch.
09 - Bake for 40 to 45 minutes until the top is golden brown and the edges are bubbling. Remove from oven and let stand for 10 minutes before serving to allow layers to set.