1. Heat oven to 350 degrees . Place potatoes in a large pot and cover with cold water. Bring to a boil over high heat; cook, boiling, for 12 minutes. Drain and return to pot.
2. Meanwhile, heat oil in a large, lidded 3 to 4 qt flameproof dish over mediumhigh heat. Crumble in turkey and cook, breaking apart with a wooden spoon, for 4 minutes. Stir in carrots, parsnips, ginger, tsp of the salt, the cumin, coriander, cinnamon, turmeric, cayenne and 1/2 cup water. Cover and cook over medium heat, stirring once or twice, for 7 minutes, adding more water if necessary. Uncover and stir in peas. Cook for 2 minutes, then remove from heat.
3. Mash potatoes in pot with milk, butter and remaining 3/4 tsp salt. Spread potatoes over turkey mixture in dish and sprinkle with cayenne, if desired. Bake, uncovered, at 350 degrees for 20 minutes.