Coat a baking dish with vegetable spray. Place one layer of sliced sweet potatoes, then one layer of new potatoes. Sprinkle with the butter substitute, 1 tablespoon of onion, a sprinkle of pepper, and repeat this layering until all potatoes are used. Pour the skim milk over the potatoes. Cover, and bake at 375� for 40-45 minutes or until potatoes are done. Uncover and bake an additional 10 minutes. Garnish and serve with fresh parsley.