Pierce the roast at intervals and insert the pancetta and garlic. Put it in a nonreactive bowl with the wine, vinegar, carrots, onions, and celery. Cover and refrigerate for 12 to 24 hours, turning once.
Lift out the beef, pat it dry, and strain the marinade, reserving both liquid and vegetables.
Braise:
Heat the oil in a Dutch oven and brown the roast on all sides. Remove it, then cook the reserved vegetables for 8 minutes. Stir in tomato paste.
Return the beef and add reserved marinade, stock, prunes, bay, rosemary, nutmeg, salt, and pepper. Cover and braise at 325 F (165 C) for about 3 hours until sliceable and tender.
Rest the beef. Discard bay and rosemary, blend the sauce smooth, and simmer until lightly thickened. Slice the beef across the grain and warm it gently in the sauce.
Make Gnocchi:
Boil the potatoes whole until tender, peel while warm, and pass through a ricer. Cool for 10 minutes.
Mix with the flour, egg, and salt just until a soft dough forms. Roll into ropes, cut irregular pieces, and mark lightly with a fork.
Boil in salted water until the gnocchi float, then cook 1 minute more. Serve beside the sliced pasticada and sauce.
Notes
Do not overwork gnocchi dough or it will become tough.
Pasticada is traditionally made ahead; chill the roast in its sauce, slice cold, and reheat gently for clean slices.