For the Meatballs:

1. Combine fresh breadcrumbs and milk in a large bowl. Allow crumbs to soak up milk.
2. Add beef, pork, cheese, garlic, chili flakes, eggs, egg yolk, and salt to bowl. Gently mix, being careful not to overwork the meat.
3. Form meat into 18 meatballs roughly the size of golfballs, and arrange on large baking sheet. Lightly sprinkle fine dry breadcrumbs on both sides.
4. Chill for 1 hour and up to one day to help the meatballs keep their shape while cooked.
5. Heat oil in large skillet over medium heat, and add meatballs. Brown on all sides, roughly eight minutes total. Lower temperature if needed to avoid burning.
6. Transfer cooked meatballs to large plate and cover to keep warm. Drain skillet, saving 1/4 cup leftover fat and oil.

For the Sauce:

1. Heat leftover fat and oil from meatballs in sauce pot over medium heat. Add onions and cook until soft and translucent, about 10 minutes. Add garlic and cook additional one minute.
2. Add tomato paste and stir while cooking until caramelized and absorbed, about two minutes.
3. Stir in crushed tomatoes, oregano, thyme, parsley, rosemary, salt, sugar, chili flakes, and vinegar.
4. Bring to a boil, reduce heat to low, and simmer until sauce thickens, about 25 to 30 minutes.

Final Steps:

1. Add meatballs to sauce, return to simmer, and cook until meatballs are fully cooked through and absorb sauce, about 20-25 minutes. Season with salt and pepper to taste.
2. As meatballs simmer in sauce, cook spaghetti until al dente. Drain and add to pot with meatballs and sauce, or parcel out individual portions on plates. Garnish with cheese and basil, and dive in.