Cook ground beef and onion in a 12-inch skillet over medium heat until beef is browned and onion is tender; about 8 minutes. Add garlic. Cook and stir until fragrant, about 1 minute; drain fat.
Add beef mixture and frozen mixed vegetables to a 5- to 6-quart slow cooker.
Whisk together beef broth, tomato paste, Worcestershire sauce, tapioca, Italian seasoning, salt, and pepper in a small bowl. Pour over beef mixture; stir to combine.
Cover and cook on Low for 4 to 6 hours or on High for 3 hours.
Prepare mashed potatoes according to package directions. Stir in cheese. Spread atop the beef mixture.