Place the cauliflower florets, broccoli florets, diced onion, chicken broth, garlic powder, salt, and pepper into the slow cooker.
Cover and cook on low for 6 hours until the vegetables are very soft.
Use an immersion blender to puree about half the soup, leaving some broccoli and cauliflower pieces intact for texture.
Stir in the shredded cheddar cheese until fully melted.
Add the milk and stir until the soup is heated through and smooth.