One hundred meatballs may seem like a lot, but don't worry—they freeze well, are relatively small, and their popularity guarantees that they will all be eaten, whether as a party snack or a main course.
Meatballs: Preheat oven to 375 F. Combine the pork, veal, and beef in a large mixing bowl, being careful not to overmix. In another bowl, whisk together the eggs, half-and-half, nutmeg, salt, and pepper.
Meanwhile, heat olive oil in a large skillet over medium heat. Add the chopped onions and, when they begin to sweat, add the minced garlic. Cook, stirring often, until the onions are soft and translucent, about 15 minutes. Do not let them color. Season to taste with salt and pepper. Remove from heat and let cool(you can do this quickly by spreading them on a sheet pan).Make a well in the bowl of ground meat, and add the half-and-half mixture in two parts, folding together gently.
Make breadcrumbs by pulsing the bread in a food processor. Add breadcrumbs and cooled onions to the meat mixture. Season with salt, pepper, and any additional nutmeg, and gently fold until thoroughly mixed.Wet your hands with cold water to minimize sticking, and roll the mixture into 1-ounce meatballs. They will be very soft but will hold their shape when cooked. Arrange on sheet pans, spacing them about 1 inch apart. Bake until just cooked, about 20 minutes.
Marinara: Melt butter in a large, heavy pot over medium heat. Add onions and cook until soft and translucent, about 15 minutes. Season with salt and pepper. Do not let brown. Add garlic, tomatoes, parsley, rosemary, and bay leaf, and season with salt, pepper, and nutmeg. Reduce heat and gently simmer for about 20 minutes. Remove bay leaf and rosemary and adjust the seasoning before serving. This recipe will yield 2–3 cups of sauce.
Assembly: Layer a serving platter with about half the sauce. Lightly coat the cooked meatballs in sauce, and arrange on the platter. Drizzle with olive oil, stud each with a toothpick, and serve.