1. In a mixing bowl, combine flour, salt, and oil. Gradually add water and knead into a firm dough. Cover and set aside.
2. Heat oil in a pan, add cumin seeds, and let them splutter. Add mashed potatoes, peas, garam masala, coriander powder, turmeric powder, and salt. Mix well and cook for a few minutes. Let it cool.
3. Divide the dough into small balls. Roll each ball into a thin oval shape. Cut it into two halves.
4. Take one half, fold it into a cone shape, and seal the edge with water. Fill the cone with the potato mixture and seal the top edge.
5. Heat oil in a frying pan. Deep fry the samosas until golden brown. Drain on paper towels.