Place the frozen broccoli florets, diced onion, minced garlic, chicken broth, shredded carrots, salt, and black pepper into the slow cooker.
Stir once to combine, then cover and cook on low for 6 hours or on high for 3 hours until the broccoli is very tender.
Turn the slow cooker to warm. Stir in the shredded cheddar cheese a handful at a time until fully melted.
Pour in the heavy cream and stir until the soup is smooth and creamy. Serve right away.