1. To make the stuffing: Heat oil in a large pan over medium heat. Add onions and sauté until very deeply browned, about 45 minutes. Season with salt, then add brown sugar and balsamic vinegar or wine. Cook another three to four minutes, until the mixture is cohesive. Allow to cool slightly.
2. Preheat oven to 300 degrees Fahrenheit (150 degrees Celsius).
3. Stuff the veal with the onion mixture. Season the veal with salt and pepper, then spread the crushed garlic over it. Place in a 9×13-inch (23×33-centimeter) baking pan. Add chicken stock to the pan and cover tightly.
4. Cook for four to five hours, or until very tender.