Place the cubed potatoes, sliced mushrooms, diced onion, and minced garlic into the slow cooker.
Add the chicken broth, dried thyme, salt, pepper, and butter. Stir once to combine the ingredients evenly.
Cover and cook on low for 6 hours until the potatoes are very soft.
Use an immersion blender to puree the soup directly in the slow cooker until smooth.
Stir in the heavy cream and let the soup warm for 10 more minutes on low.
Ladle into bowls and top each serving with a fresh thyme sprig.