Grate the bread or pulse into crumbs in a food processor. In a small bowl toss the breadcrumbs with 1/3 cup cold water to re-hydrate.
In a large bowl, combine the breadcrumbs, pork, veal, beef, Parmesan, Pecorino, parsley, salt, onion, garlic, and egg and mix until combined. Season the meat mixture with pepper.
Using your hands, gently form the meat mixture into 18 slightly larger than golf ball-sized balls. (Packing the meat mixture too tightly together will result in tough meatballs). Refrigerate for at least an hour or up to 24.
Heat half the oil in a large non-stick skillet over medium-high heat. Add half of the meatballs and cook, turning occasionally, until well browned on all sides, about 6 minutes. Transfer the meatballs to a plate. Drain the oil and wipe out the skillet, return to the heat, and repeat with the remaining oil and meatballs.
Drain and wipe out the skillet again. Return all the meatballs to the skillet and pour in the marinara sauce. Bring to a boil, lower the heat, and simmer, covered, swirling the pan occasionally, until the meatballs are cooked through about 15 minutes. The cheese in the meatballs will start to melt when the meatballs are ready. Serve immediately with spaghetti or on sandwiches. These meatballs can be stored, covered in the refrigerator for 3 days, or frozen for up to 6 weeks.
Heat the oil in a medium saucepan over medium-high heat. Saute the onion and garlic, stirring, until lightly browned, about 3 minutes. Add the tomatoes and the herb sprigs and bring to a boil. Lower the heat and simmer, covered, for 10 minutes.
Remove and discard the herb sprigs. Stir in the salt and season with pepper to taste. Use now, or store covered in the refrigerator for up to 3 days, or freeze for up to 2 months.
If your bread is fresh, dry it out in at 350 degree F for 10 to 15 minutes.
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