Heat the oil in a large skillet over medium heat. Add the onions and cook until softened, 6 to 8 minutes, stirring often to cook evenly.
Put the potatoes, carrots, garlic, and herbs in the slow cooker; add the onions and oil, scraping them out of the pan. Add enough of the water or broth to cover everything. Cover and cook on HIGH for 1 hour.
Turn the cooker to LOW and cook until the vegetables are soft, 5 to 7 hours. Puree in batches in a food processor or right in the slow cooker with a handheld immersion blender; the soup will be nice and thick. Stir in the honey and grate the nutmeg right over the crock. Season with salt and pepper. Keep warm on LOW without letting it come to a boil until serving. Ladle the hot soup into bowls and enjoy.