Prepare the onions by peeling them while keeping the roots and stems intact. Cut halfway into the centre. Place onions in a large pot, cover well with cold water, bring to the boil, and boil briskly for between 45 minutes to 1 hour until tender.
While the onions are cooking, peel and boil the potato until fork tender. Drain and mash it; set aside.
In a small frying pan, melt 15 g of butter and sauté 4 tablespoons of finely chopped onion until soft and translucent.
In a large bowl, combine the sautéed onions, minced meat, mashed potato, parsley, breadcrumbs, cream, egg, pepper, salt, and optional spices. Knead vigorously with both hands or beat with a wooden spoon until all the ingredients are well blended and the mixture is smooth and fluffy. This is Swedish Meatball mix.
Once the onions have cooled, carefully remove the core and separate the onion layers. Discard the inner part of the onions (or save them for some other purpose if the leaves are too small to stuff). Cut the largest outer layers of the onions in half but remember to leave them large enough to enclose the stuffing.
Place a heaped teaspoon of meat mixture in the centre of each onion layer, fold or roll over the edges to enclose the filling. (At this point they may be covered with plastic wrapping and refrigerated for up to 2 days before cooking).
Preheat your oven to 200°C/400°F (fan) or 220°C/430°F (no fan). Place a shallow roasting pan in the preheated oven with 60 grams of butter to heat up until sizzling.
Carefully place the onion rolls in the hot butter, using a brush to baste the tops of the rolls with the melted butter in the pan.
Bake for 15 minutes, then baste with the butter again, sprinkle with fresh breadcrumbs, (and optionally you could drizzle a little more melted butter over the breadcrumbs) and return to the oven for another 15 minutes or until golden brown. The onion rolls may require a further 5-10 minutes of cooking time if your rolls are big.
Serve warm and enjoy!