Prepare the meatballs: In a large bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, egg, minced garlic, chopped parsley, salt, and black pepper. Mix well until all ingredients are evenly incorporated.

Shape the meatballs: Roll the mixture into 1.5-inch balls. You should get about 20 meatballs. Place them on a tray and refrigerate for 15 minutes to firm up.

Cook the meatballs: Heat olive oil in a large skillet over medium heat. Add the meatballs and brown them on all sides, about 8-10 minutes total. Work in batches if needed to avoid overcrowding. Remove and set aside.

Prepare the sauce: In the same skillet, add marinara sauce and red pepper flakes if using. Bring to a simmer, then return the meatballs to the skillet. Cover and simmer for 15-20 minutes, stirring occasionally.

Cook the spaghetti: While the meatballs simmer, bring a large pot of salted water to a boil. Add spaghetti and cook according to package instructions until al dente. Drain, reserving 1/2 cup of pasta water.

Combine and serve: Toss the cooked spaghetti with a little of the reserved pasta water to loosen it up. Serve the spaghetti topped with meatballs and sauce. Garnish with extra Parmesan cheese and parsley if desired.