Preheat the oven to 400°F. Line a baking sheet with parchment paper.
In a large bowl, mix together the ground beef, egg, bread crumbs, Parmesan cheese, garlic, salt, black pepper, Italian seasoning, and red pepper flakes (if using).
Roll the mixture into balls about the size of a golf ball. Place the meatballs on the prepared baking sheet.
Bake the meatballs for 15-20 minutes, or until they are cooked through and browned on the outside.
Meanwhile, bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions.
In a large saucepan, heat the olive oil over medium heat. Add the marinara sauce and simmer for 5-10 minutes, or until heated through.
When the spaghetti is cooked, drain it and add it to the saucepan with the marinara sauce. Toss to coat the spaghetti in the sauce.
Add the cooked meatballs to the saucepan with the spaghetti and toss to combine.
Serve the meatballs and spaghetti hot, with additional Parmesan cheese and fresh parsley if desired.