Grate onion using a box grater into a bowl until you have about 1/2 cup with juices. Save the rest of the onion for later. Add bread to the bowl with the onion and press the onion into the bread to help the bread soak up the onion juices. Add ground pork and ground sirloin to the bowl, along with the egg, parmesan cheese, chopped parsley, salt and pepper. Mix together with your hands until well-blended, trying to break up the bread as much as possible.
Using a scoop, form the balls into even sizes and place on a tray or plate. When you're done, set a large skillet or braising dish over medium-high heat. Drizzle with olive oil. Once hot. Place meatballs in the pan and cook for 2 minutes or until bottoms have browned. Flip and cook another few minutes to brown the other side. Remove when browned on the outside but not fully cooked inside. Set aside
For sauce:
Before you start on the sauce, boil and salt water. Add spaghetti and cook until al dente, about 8 minutes. Drain pasta. Add back to pot and toss with some with olive oil so it doesn't stick together. Set aside.
Add more olive oil to your skillet or braising dish. Add onions and cook until tender and translucent, about 4-5 minutes. Add garlic and cook for 1 more minute until fragrant. Add passata, water, herbs, and spices and stir to remove any browned pieces from the pan. Bring to a boil and then reduce to a simmer.
Add meatballs to the sauce and cover to cook the sauce and meatballs together for 5 minutes. Remove cover and stir. Cook for 5 more minutes or until onions are cooked and sauce is thickened. Remove from heat and serve with spaghetti.