01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish with a little butter.
02 - Place the potatoes in a large pot, cover with cold water, add 1 tsp salt, and bring to a boil. Simmer for 18-20 minutes until just tender but not falling apart. Drain, cool, and slice into ¼-inch rounds.
03 - Place eggs in a saucepan, cover with cold water, and bring to a boil. Simmer for 10 minutes. Drain, cool under cold water, peel, and slice.
04 - In a skillet, melt 1 tbsp butter and gently sauté the onion until translucent; set aside.
05 - Layer one-third of the potato slices on the bottom of the baking dish. Sprinkle with a little salt and pepper.
06 - Add half of the egg slices, half of the sausage slices, and half of the sautéed onions. Dollop and gently spread one-third of the sour cream over the layer.
07 - Repeat layers: potatoes, remaining eggs, sausage, onions, and another third of the sour cream.
08 - Finish with a final layer of potatoes, season with any remaining salt and pepper, and spread the remaining sour cream over the top. Dot with butter. Optionally, sprinkle breadcrumbs over the top for extra crunch.
09 - Bake for 40-45 minutes until the top is golden and bubbling. Let stand 10 minutes before serving.