In a large mixing bowl, combine the ground beef, breadcrumbs, grated Parmesan, chopped parsley, minced garlic, egg, salt, and black pepper. Mix gently until combined, avoiding overmixing.
Heat a tablespoon of olive oil in a large skillet over medium heat. Shape the meat mixture into golf ball-sized meatballs and place in the skillet. Cook in batches, turning occasionally, until browned and cooked through, about 8-10 minutes. Remove and set aside.
Bring a large pot of salted water to a boil. Cook the spaghetti or preferred pasta according to package instructions until al dente. Drain and set aside.
Return the skillet to medium heat and pour in the jar of marinara sauce. Add the cooked meatballs back, stirring gently to coat. Let simmer for 5 minutes to meld flavors.
Divide the pasta into serving bowls. Spoon meatballs and marinara over the top. Sprinkle shredded mozzarella cheese on each bowl. Optionally, place bowls under a broiler for 2-3 minutes until cheese is bubbly and golden.
Garnish with extra chopped parsley or Parmesan if desired. Serve hot and enjoy!