Place a skillet over medium-high heat. Add the ground beef and chopped onion. Cook until the beef is fully browned and the onion is translucent. Drain the excess grease thoroughly.
Transfer the cooked beef mixture to a 6-quart slow cooker.
Add the diced potatoes, beef broth, undrained diced tomatoes, dried parsley, salt, and black pepper to the slow cooker. Stir to combine.
Cover and cook on LOW for 7-8 hours or on HIGH for 3-4 hours, until the potatoes are tender.
About 30 minutes before serving, whisk the cornstarch into the milk until smooth (no lumps remain). Pour this mixture into the slow cooker and stir gently.
Stir in the shredded cheddar cheese until melted and combined.
Cover and cook on HIGH for an additional 20-30 minutes until the soup has thickened and is creamy.
Serve hot, garnished with extra cheese, parsley, or crispy bacon if desired.