Place the broccoli florets, diced onion, chicken broth, garlic powder, salt, and black pepper into the slow cooker.
Cover and cook on low for 5 hours until the broccoli is very soft.
Stir in the heavy whipping cream and cream cheese cubes until the cream cheese melts into the broth.
Use an immersion blender to puree the soup until smooth, or mash with a potato masher for a slightly chunkier texture.
Add the shredded cheddar cheese a handful at a time, stirring after each addition until fully melted and the soup thickens.