In a mixing bowl, combine all-purpose flour, salt, and oil. Gradually add water and knead into a firm dough. Set aside.
Heat oil in a pan. Add cumin seeds and let them splutter. Add mashed potatoes, green peas, garam masala, coriander powder, chili powder, and salt. Mix well and cook for 5 minutes. Let it cool.
Divide the dough into small balls. Roll each ball 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 samosas on a baking sheet and bake for 20-25 minutes or until golden brown.