In a mixing bowl, combine mashed potatoes, chopped onion, chopped cilantro/coriander leaves, salt, pepper, and half of the butter.
Mix until well combined.
Shape the mixture into round flat patties.
Heat the remaining butter in a frying pan or skillet over medium-low heat.
Fry the potato patties for about 5 minutes on each side, or until golden brown and crispy.
Serve the hot potato patties immediately with tomato sauce, optionally between toasted hamburger buns.