1. Combine breadcrumbs and milk in a large bowl. Let sit for a few minutes to soften.

2. Add ground beef, ground pork, egg, Parmesan, garlic, parsley, 1 teaspoon salt, and 1/4 teaspoon pepper to the breadcrumb mixture. Mix gently until just combined.

3. Scoop heaping tablespoonfuls of the mixture and roll into 24 meatballs. Set aside.

4. Heat 1 tablespoon olive oil in a large skillet or Dutch oven over medium heat. Brown the meatballs in batches, about 5–7 minutes, turning to brown all sides. Remove to a plate.

5. Add the remaining tablespoon olive oil and chopped onion to the same pot. Sauté until softened, about 5 minutes. Stir in garlic and cook 1 more minute.

6. Add tomato paste and cook for 1 minute. Stir in crushed tomatoes, oregano, basil, 1/2 teaspoon salt, 1/4 teaspoon pepper, and sugar if using. Bring to a simmer.

7. Gently add the browned meatballs and any juices to the sauce. Simmer, partially covered, for 30 minutes, stirring occasionally.

8. Meanwhile, cook spaghetti in a large pot of salted boiling water until al dente. Reserve 1/2 cup pasta water, then drain.

9. If sauce is too thick, add reserved pasta water to desired consistency. Toss some sauce with the spaghetti and divide among plates.

10. Top with meatballs, extra sauce, and sprinkle with Parmesan and parsley. Serve hot.