Shepherd's Pie With Potato Topping

Make the potato topping: In a large saucepan, combine the potatoes with enough cold water to cover by 1 inch. Bring to a boil and cook until tender, about 15 minutes. Drain the potatoes well in a colander. In a large bowl, combine the potatoes, milk, butter, salt, and pepper to taste. Using an electric mixer, beat on high speed until smooth. Set aside. Preheat the oven to 350 F. Bring a small saucepan of lightly salted water to a boil. Add the carrots and cook for 2 minutes to blanch. Drain and set aside. Heat the oil in a large skillet over medium-high heat. Add the onion and cook, stirring, until softened, about 2 minutes. Add the garlic and cook for 15 seconds. Stir in the ground turkey, breaking up the large pieces with a fork, and cook until browned. Add the blanched carrots, peas, corn, tomatoes, and tomato paste. Mix well. Cook, stirring often until the flavors have blended, about 10 minutes. Season with black pepper to taste. Spray a 3-quart gratin dish or shallow casserole with vegetable oil spray. Spoon the vegetable-turkey mixture into the prepared dish. Top with dollops of mashed potatoes. Bake for 30 to 45 minutes, until heated through. Serve hot.