Prepare the bollito: Place beef, celery, carrot, clove-studded onion, bay leaf, and juniper in a pot with cold water. Bring to a boil, salt, and simmer for ~3 hours, skimming as needed.
Grind the meat: Drain the beef and pass through a meat grinder.
Toast the bread: Cube fresh bread, drizzle lightly with oil, and toast until dry and golden. Once cooled, soak in milk and squeeze well.
Second grind: Pass the beef, soaked bread, and mortadella together through the grinder.
Mix: Gather in a bowl. Add egg, cheese, lemon zest, salt, pepper, and nutmeg. Mix into a soft but workable dough, adding breadcrumbs if needed.
Shape: With wet hands, form oval patties, slightly flattened. Dredge lightly in breadcrumbs.
Cook: Melt butter with rosemary in a pan. Fry mondeghini gently until golden and crisp outside.
Serve: Plate hot with fresh salad.
