In a small bowl, mix together the milk and bread crumbs.
To a medium bowl, mix together the ground beef and ground pork. Sprinkle the ground meat with salt and give it another mix.
In a medium sauté pan, placed over medium heat, add a tablespoon of olive oil. Carefully add the diced onion to the oil and cook until translucent and soft, about 5-7 minutes, being sure mix them every few minutes or so. Turn the heat down to medium-low, and add the minced garlic, Italian parsley, sage and crushed red pepper. Add the onion mixture to the bowl of meat. By this time, the whole milk and bread crumbs should be moistened; add it to the meat mixture, along with the grated parmesan and egg. Mix the meat mixture until completely combined, but being careful to not over mix. Using your hands, roll a few tablespoons (or 2 ounces) into a ball. Repeat until all meatballs are formed.
To sear the meatballs, add a few tablespoons of oil to the sauté pan (you can use the same one you just used--we're into saving dishes around here). When the oil is hot, gently add the meatballs and sear on all sides for about 1-2 minutes per side, until golden brown. Transfer them to a bed of paper towels to drain. Repeat until you've seared all of the meatballs, adding more oil if the pan needs it.
To make the tomato sauce, add the olive oil to a dutch oven or medium pot. When the oil is hot, add the piece of onion and sear on both sides for 1-2 minutes, until lightly golden brown. Pour in the can of tomatoes, add the parmesan rind (if using), crushed red pepper, butter and sage bundle; mix, cover the pot and cook the tomato sauce over low heat for 30 minutes and until the kitchens smells awesome.
Add the meatballs to the tomato sauce and cook for an additional 10-15 minutes. To serve, toss the cooked pasta with a few big spoonfuls of sauce. Divide the pasta between bowls, top with a few meatballs and garnish with a sprinkling of Italian parsley and some Parmesan-Reggiano.