Boil, peel and mash the potato.
In a wok or kadai, heat a tablespoon of oil. Temper the mustard seeds.
Add the chopped onions and fry till transparent.
Add the ginger garlic paste and fry till the raw smell is gone.
Add the mashed potato, the spice powders and salt. Stir to mix well on a low flame for 2 min, then turn off the gas.
Cut the sides of the bread and immerse each slice in water for just a second. Squeeze out the water by pressing the bread in between your palms.
Place the potato filling inside the bread and cover the filling completely.
If you like, roll the filled bread over breadcrumbs to coat all over. Deep fry the rolls on a medium heat till brown.