Bring kabocha squash, onion, and garlic to a boil in 4 cups of water or stock. Simmer at a low boil until squash is completely soft, about 15 minutes. Mash the squash, onion, and garlic with a potato masher in the soup pot. Add the spinach and cook for just a couple more minutes. Season to taste with sea salt and pepper. Garnish with organic sour cream, mozzarella cheese, or crumbled feta.