Brown the beef: In a skillet over medium heat, cook ground beef until no longer pink. Drain excess fat.
Sauté aromatics: Add onion and garlic to the skillet; cook until softened and fragrant.
Combine in slow cooker: Transfer beef, onion, and garlic to the slow cooker. Add diced tomatoes, tomato sauce, beef broth, Italian seasoning, salt, and pepper. Stir to combine.
Cook: Cover and cook on low for 6–8 hours or on high for 3–4 hours.
Add tortellini: About 30 minutes before serving, stir in tortellini and cook until tender.
Serve: Ladle into bowls and garnish with fresh basil or parsley if desired.
