Put sweet potato into a saucepan with enough water to cover completely; bring to a boil. Reduce heat to medium-low and simmer until just tender, about 15 minutes; drain.
Preheat oven to 400 degrees F (200 degrees C).
Combine water, onion, and garlic in a saucepan; bring to a boil, reduce heat to low, and cook at a simmer until onion softens, about 5 minutes.
Stir gravy mix, cumin, cinnamon, curry powder, ginger, black pepper, turmeric, and salt into the water; add leftover pot roast, sweet potatoes, peas, and corn. Cook the mixture, stirring occasionally, until heated through, about 5 minutes; pour into the frozen pie crust. Sprinkle feta cheese over the filling.
Bake in preheated oven until crust is golden brown around edges, about 25 minutes.
