Cook the onions in the butter over medium heat until they are soft and translucent. Add the garlic and cook another minute.Pour in the stock and add the thyme, nutmeg, salt, fish and nettles. Bring this to a gentle simmer and cook for 15-20 minutes. You want the nettles to be very tender.When the nettles are ready, taste the soup and add salt and nutmeg if it needs it. You want to be able to pick up the nutmeg in the final dish.Puree the soup in a blender. You might need to do this in batches. If you want to get fancy, push the puree through a fine-meshed strainer. Put back on the stove just to warm through. Serve with a dollop of sour cream and some black pepper.