Brown the Italian sausage in a skillet over medium heat, breaking it into crumbles as it cooks. Drain excess fat and transfer the sausage to the slow cooker.
Add the sliced mushrooms, diced onion, diced carrots, minced garlic, pearl barley, beef broth, dried thyme, bay leaves, salt, and black pepper to the slow cooker.
Stir everything together, cover, and cook on low for 6 to 7 hours until the barley is tender and the vegetables are soft.
Remove the bay leaves before serving. Ladle the soup into bowls and top each with chopped parsley and grated Parmesan cheese.