Scrub potatoes and boil in lightly salted water for about 30 minutes or until potatoes are tender. Drain and allow potatoes to cool slightly before handling. Peel and dice potatoes into ¼ inch cubes.
Heat vegetable oil in a medium pan. Add onions and fry until soft and translucent. Add cumin seeds and mustard seeds. When seeds start to sputter, stir in garam masala, coriander powder, and chili powder.
Add frozen peas and cubed potatoes. Season with salt. Give it a good stir. Remove and allow filling to cool.
Place one sheet of fillo pastry on a work surface. Brush lightly with vegetable oil. Fold into half lengthwise. Place 1½ to 2 tablespoons filling at one corner. Fold a corner of pastry over filling to create a triangle. Continue folding until entire strip of pastry is used up and a triangular parcel is formed. Repeat until all filling is used up.
Place seam side down on a lightly oiled baking tray and brush the tops of pastry parcels lightly with oil. Bake in a 350°F oven for about 30 minutes or until golden brown. Remove and serve immediately.