01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter.
02 - Place potatoes in large pot, cover with cold water, add 1 tsp salt, bring to boil. Simmer 18–20 minutes until tender but firm. Drain, cool, slice into ¼-inch rounds.
03 - Place eggs in saucepan, cover with cold water, bring to boil. Simmer 10 minutes. Drain, cool under cold water, peel, and slice.
04 - Melt 1 tbsp butter in skillet, gently sauté onion until translucent. Set aside.
05 - Layer one-third potato slices on bottom of baking dish. Season lightly with salt and pepper.
06 - Top with half the egg slices, half the sausage, and half the onions. Spread one-third sour cream over layer.
07 - Repeat with remaining potatoes, eggs, sausage, onions, and another third of sour cream.
08 - Finish with final potato layer, season with remaining salt and pepper, spread remaining sour cream. Dot with butter and sprinkle breadcrumbs.
09 - Bake 40–45 minutes until golden and bubbling. Let stand 10 minutes before serving.