Set a large 7-8 quart saucepot over medium heat. Add the butter to the pot. Once melted, add the onion wedges. Sauté the onion wedges for 5-8 minutes, to caramelize, soften, and release their juices.
Add the tomato sauce. Partially cover the pot and simmer on medium-low for 45-60 minutes.
Methodically and Delicately Make and Bake the Meatballs:
Meanwhile, set out a large mixing bowl. Combine the breadcrumbs and milk in the bowl. Then add in the minced garlic, Italian seasoning, eggs, grated parmesan, parsley, 2 ½ teaspoons salt, and ½ teaspoon ground black pepper. Mix well.
Add in the ground beef, veal, and Italian sausage. Gently break the ground meat into small chunks with your fingers. Then toss it with the milk and herb mixture to coat the meat chunks. This pre-mixing process helps to get the ingredients evenly distributed, without overmixing.
Now mix the meatball mixture with your hands until evenly combined, but not a moment longer. (Overmixing meatballs makes them tough! You want the mixture to be even, but not packed down.)
Preheat the oven to 400 degrees F. Set out a large rimmed baking sheet and line it with parchment paper.
Use a 2 ½ - 3 tablespoon scoop to portion out the meatballs. Roll them well, but do not press and pack them tightly. If needed, use your fingers to pinch any cracks together. Set them on the baking sheet. (At this point, you could cover and chill the meatballs for up to 2 days. Or you could freeze them.)
Bake the meatballs for 20 minutes to set. (The meatballs will finish cooking in the tomato sauce. If you plan to use the meatballs in a different way, bake them for 25 minutes to fully cook.)
Simmer the Meatballs in the Tomato Sauce:
Now use tongs to pull the chunks of onion out of the tomato sauce. Discard the onions. Then use tongs to move the meatballs into the tomato sauce. Pour the pan juices into the sauce and gently stir. Simmer for 10-15 minutes. Serve warm.
Once cooked, you can freeze the meatballs and sauce for up to 3 months.