In a saucepan combine potatoes with 1 cup of the broth. Bring to a boil and reduce heat. Simmer covered for about 10-15 minutes or until potatoes are tender. Remove from heat, place in a bowl and cool slightly. In a large saucepan, over medium high heat, melt the butter. Add the sliced leeks and celery, cook for about 4 minutes or until tender. Add the remaining broth, (sweet potato and nutmeg) and bring to a boil, reduce heat and simmer, covered, for about 10 minutes. Transfer potato mixture in a blender or food processor and blend or process until smooth. Stir in potato mixture, (milk) and season with salt and pepper. Cook and stir occasionally for another 5 minutes, until thickened and warmed through. Season to taste with salt and fresh ground pepper. Garnish with a fresh herb spring of your choice, I used parsley.