Add the sweet potato cubes, cauliflower florets, diced onion, and minced garlic to the slow cooker.
Sprinkle the curry powder over the vegetables, then pour in the vegetable broth and stir everything together.
Cover and cook on low for 7 hours or on high for 3 to 4 hours, until the sweet potatoes and cauliflower are very soft.
Stir in the coconut milk, salt, and black pepper.
Use an immersion blender to partially puree the soup for a creamy texture, or leave it chunky if you prefer.
Ladle into bowls and top each serving with a sprinkle of fresh cilantro.