Place the sliced Italian sausage in a large skillet over medium heat. Cook until browned on all sides, about 5 minutes, then transfer the sausage to the slow cooker.
Add the diced onion, minced garlic, sliced carrots, and quartered mushrooms to the same skillet. Cook for 4 to 5 minutes until the vegetables begin to soften.
Transfer the vegetables to the slow cooker. Add the diced tomatoes with their juices, beef broth, oregano, and basil.
Season with salt and black pepper. Stir everything together, cover, and cook on low for 6 to 7 hours.
Taste and adjust seasoning if needed. Stir in the chopped fresh parsley just before serving.