Add diced potatoes, onion, and chicken broth to a slow cooker.
Cover and cook on low for 6–7 hours or until potatoes are tender.
In a small saucepan, melt butter over medium heat. Stir in flour and cook for 1–2 minutes to form a smooth paste.
Slowly whisk in milk and cook until slightly thickened.
Pour this mixture into the slow cooker and stir gently.
Add heavy cream, shredded cheese, salt, and pepper. Stir until combined.
Cover and cook for another 20–30 minutes until the soup is creamy and heated through.
Ladle into bowls and serve warm.
