Preheat oven to 375°.
To prepare meatballs, combine first 10 ingredients; shape mixture into 24 (3/4-inch) meatballs. Arrange meatballs 2 inches apart on a jelly roll pan. Bake at 375° for 12 minutes, turning after 6 minutes. Set aside.
To prepare sauce, heat oil in a large nonstick skillet over medium-high heat. Add 1 1/2 cups onion; sauté 2 minutes. Add 2 garlic cloves; sauté 1 minute. Add tomato paste, oregano, diced tomatoes, and bay leaves; bring to a boil. Reduce heat, and simmer 15 minutes or until slightly thick.
Combine miso and water; stir with a whisk. Add miso mixture and 1/4 teaspoon pepper to sauce. Discard bay leaves. Serve sauce over spaghetti and meatballs. Sprinkle with cheese; garnish with parsley sprigs, if desired.