Heat one tablespoon of oil in a frypan over medium heat. Add the onion, and cook until browned.
Stir in 1 cup of yoghurt, curry powder, chilli powder, and 2 tablespoons of the chopped coriander. Cook and stir until most of the liquid has evaporated. Remove from the heat, and stir in the remaining coriander.
Lay out one sheet of filo pastry and cover the rest with a damp towel. Place about 1 tablespoon of the onion coriander mixture onto the corner, not too close to the edge of the sheet of pastry, then fold up into fourths toward the opposite side to enclose the filling.
Starting with the filled end, fold towards the other end in a triangle shape, pressing the edges with moistened fingers to seal. Repeat with remaining filling and pastry sheets.
Heat 2cm of oil in a deep fryer (or deep heavy frypan over a medium heat) to 185C.
In a small bowl, stir together the remaining yoghurt, cream, and water. Dip the samosas in the yoghurt mixture, then dredge in flour, and shake off the excess.
Fry in small batches so they are not touching, for about 4 minutes, turning once. Drain on paper towels and serve warm.