Put the potatoes in a pan of cold water, bring to the boil then cook until just tender (about 7 or 8 minutes). Rinse with cold water, drain and let them cool. When cooled, remove the skin and slice them into 5mm rounds.
Cook the eggs, allow to cool, then peel and slice them too into 5mm rounds.
Skin the sausage and slice it into 5mm rounds.
Turn the oven on to 200°C.
Grease a deep, heatproof dish with some butter and scatter 1 Tbsp breadcrumbs over the base (if you're using it).
Layer the bottom with potatoes, season with salt, then add a layer of sausage and crumble some butter around. Then layer the eggs with some salt. Finally spoon over about third of the soured cream, spreading it evenly.
Repeat this layering once or twice more (depending on the size of your dish) but the top layer should be potatoes. Season with salt and pepper, pour over the sour cream and spread it evenly. Sprinkle over with breadcrumbs or grated cheese.
Put the dish into the oven and cook it for about 35-45 minutes until golden.