In a large bowl, combine ground beef, Italian sausage, breadcrumbs, Parmesan cheese, egg, garlic, oregano, basil, salt, and black pepper. Mix until well combined. Form the mixture into meatballs, about 1 inch in diameter.
In a large pot or Dutch oven, heat olive oil over medium heat. Add the meatballs in batches, cooking until browned on all sides, about 5-7 minutes. Remove the meatballs and set aside.
In the same pot, add onion, carrots, and celery. Sauté until the vegetables are tender, about 5 minutes.
Pour in the beef broth and diced tomatoes. Stir in Worcestershire sauce and red pepper flakes, if using. Bring to a simmer.
Return the meatballs to the pot and let the stew simmer for 20-25 minutes, until the meatballs are cooked through and the flavors meld together.
Stir in the frozen peas and chopped kale or spinach. Cook for an additional 5 minutes until the greens are wilted and heated through.
Taste and adjust seasoning if necessary. Serve hot.