Bring potatoes to boil in salted water to cover. Boil until tender, about 10 minutes. Drain and set aside.
Meanwhile, in large ovenproof nonstick or cast-iron skillet, heat oil over medium-high heat. Add turkey and sauté for about 5 minutes, until turkey is mostly browned. Add leek and carrot and sauté for 2 to 3 minutes, until leek is limp. Add zucchini and garlic and sauté for another minute. Stir in corn and reduce heat to low.
In small bowl, combine beef broth, tomato paste, cornstarch and herbs. Mix well. Stir into skillet. Increase heat to medium-high and cook, stirring frequently, until mixture thickens, 1 to 2 minutes. Cook for another minute, then remove from heat. Season with salt and pepper.
Preheat broiler.
Mash potatoes in medium bowl. Whip in buttermilk and butter. Add salt and white pepper.
Spoon mashed potatoes over turkey mixture in skillet, smoothing top. Place under broiler and broil until browned on top, 5 to 8 minutes. Let stand for 5 minutes before serving.