Place the potatoes and 1 tablespoon salt in a medium saucepan. Cover with water, bring to a boil, and cook for 16 to 18 minutes, until tender. Drain and return to the saucepan.
Add ½ teaspoon salt, the sour cream, ½ teaspoon turmeric, ¼ teaspoon garam masala, ¼ teaspoon cumin and ¼ teaspoon chili powder, and mash with a potato masher. Set aside.
Heat a large, deep nonstick skillet over medium heat and melt the ghee. Add the onion and cook until golden, 6 to 8 minutes. Add the garlic and ginger, and cook until fragrant, about 2 minutes.
Add the ground turkey and 1 teaspoon salt to the skillet. Brown the meat, using a wooden spoon to break it until small pieces, until cooked through, 4 to 5 minutes.
Add the tomato sauce, ¼ cup water, cilantro, chile pepper, remaining ¾ teaspoon garam masala, remaining ½ teaspoon cumin, remaining ½ teaspoon chili powder, and remaining ½ teaspoon turmeric, and stir well.
Cover and cook on low heat for 20 to 25 minutes, then stir in the frozen peas and cook for about 3 minutes more, until the peas are warmed through.
Transfer the meat mixture to an 8- or 9-inch baking dish or deep pie plate, spreading it across in an even layer. Dollop the potatoes across the top and gently push them around until they cover the entire top of the casserole, then run the tines of a fork across the top for texture. Dust with the chili powder for color.