1. Place potatoes in a large pot and cover with cold water. Salt lightly. Cover and bring to a boil. Reduce heat and simmer for 20 minutes or until tender. Drain well and mash.
2. While potatoes are cooking, bring 1-1/2 cups of water to a boil in another large pot. Add cauliflower and steam until very tender, about 15 minutes. Drain and place in a blender with the milk and margarine. Puree, in batches if necessary, until very smooth.
3. Combine potatoes and the cauliflower puree. Season with salt and pepper. Garnish with parsley if desired. Makes 8 servings.