01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with a small amount of butter.
02 - Place 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 onion until translucent; set aside.
05 - Layer one-third of potato slices on bottom of baking dish. Sprinkle with a little salt and pepper. Add half of egg slices, half of sausage slices, and half of sautéed onions. Dollop and spread one-third of sour cream over layer.
06 - Repeat with another layer of potatoes, remaining eggs, sausage, onions, and another third of sour cream.
07 - Finish with final layer of potatoes, season with remaining salt and pepper, spread remaining sour cream over top, and dot with butter. Optionally sprinkle breadcrumbs for crunch. Bake for 40–45 minutes until golden and bubbling. Let stand 10 minutes before serving.