Trim the cabbage core, boil the whole head in salted water for 10 minutes, then peel off 10 leaves as they soften. Pat the leaves dry and trim the thick ribs so they lie flat. Chop the remaining cabbage.
Mix the pork, whole onion, breadcrumbs, cooked rice, eggs, garlic, marjoram, filling salt, pepper, and half the chopped parsley until evenly combined.
Divide the filling among the leaves. Shape each portion into a short cylinder, place it near the lower third of a leaf, fold in the sides, and roll tightly with the seam underneath.
Pour 1 cup of stock into a deep pan and arrange the rolls seam-side down. Add the remaining stock, bring to a gentle simmer, cover, and cook for 25 minutes, until the filling reaches 71°C / 160°F. Remove the rolls.
Reduce the remaining stock to about 1 1/2 cups. Stir in passata, tomato paste, sauce salt, pepper, and sugar. Simmer for 5 minutes until lightly thickened, then stir in the butter.
Return the rolls to the sauce, brush a thin coating over each one, cover, and warm gently for 5 minutes. Scatter with the remaining parsley and serve.