Preheat oven to 375°F (190°C).
In a pan, heat oil and add cumin seeds. Once they splutter, add mashed potatoes, peas, garam masala, coriander powder, and salt. Mix well and cook for 5 minutes. Set aside to cool.
In a mixing bowl, combine flour, salt, and oil. Gradually add water and knead into a firm dough. Cover and let it rest for 15 minutes.
Divide the dough into equal portions. Roll each portion into a thin oval shape. Cut it in half to form two semi-circles.
Take one semi-circle, fold it into a cone shape, and seal the edge with water. Fill the cone with the potato mixture and seal the top edge.
Place the filled samosas on a baking sheet lined with parchment paper. Brush them lightly with oil.
Bake in the preheated oven for 20-25 minutes or until golden brown and crisp.