Place peeled and chopped potatoes in a large pot of boiling water. Bring to a boil over high heat for 10 minutes. Drain and return to pot.
Mash the potatoes with butter, milk, and 1 tsp. salt.
Heat oil in a large baking dish over medium-high heat. Add the ground turkey and cook, breaking apart with a wooden spoon, for 4-5 minutes. Stir in the mirepoix (carrots, celery, onions) along with the ginger, 1 tsp. of the salt, curry, gram masala, turmeric, and 3/4 cup water.
Cover and cook over medium heat, stirring once or twice, for about 6-7 minutes, adding more water if necessary. Uncover and stir in the peas. Cook for 2 minutes, then remove from heat.
Spread mashed potatoes over turkey mixture in a baking dish, and sprinkle with gram masala on top, if desired. Bake, uncovered, at 350 degrees for 20-25 minutes.