In a medium bowl, combine the beef, egg, breadcrumbs, Parmesan cheese, onion, garlic, Italian seasoning, salt, and pepper. Mix until just combined.
With a meatballer or a medium spring-loaded cookie scoop, scoop a rounded ball of the meat, about 1 ½ tablespoons, and form into meatballs. Repeat with the remaining meat mixture. You should get about 20 meatballs. Heat the oil in a large pot over medium heat. Add the meatballs and cook for 5–7 minutes, turning occasionally, until browned on all sides. Remove the meatballs from the pot and set aside.
Add the oil to the same pot. Add the onion and red bell pepper and cook for 4–5 minutes, until softened. Add garlic and cook 1 more minute, until fragrant.
Stir in the crushed tomatoes, vegetable broth, salt, Italian seasoning, and crushed red pepper flakes. Return the browned meatballs to the pot and bring to a boil.
Stir in the pasta and cook uncovered for 7–9 minutes until tender. During the last 2 minutes of cooking, add the chopped kale and cook until wilted.
Serve hot, garnished with grated Parmesan cheese.