Preheat oven to 425 degrees. In a large pot, add potatoes and turmeric, and fill with water. Bring to a boil, turn down heat to a simmer, and cook for about 5 additional minutes. Drain and dry potatoes, and add curry powder, garam masala, salt, pepper, and mustard seeds. On a large foil-lined baking sheet, spread out potatoes. Cook for 20-30 minutes, flipping once.