In a bowl add mashed potatoes. Mash the beans and add to the potaotes.
Add fried onions, garlic powder, pepper, and salt. MIx well, Add rice flour or plain flour gradually to form a dough.
Divide them equally into 10 portions. Use hands or a cookie cutter to form 1/2 inch thick patties.
Refrigerate for 30 minutes (optional). Mix the arrowroot powder and water to form a paste.
Heat the oil in a skillet over medium-high heat until almost smoking. . Coat the potato patties in the flour mixture and then into panko crumbs.
Add the potato cakes to the pan and reduce the heat to medium-low. Cook until they have formed a golden crust. Turn and cook the other side until golden brown.
Serve hot with sauce or sour cream on top.
Notes
To get a golden crust on the sides of the black bean and potato cake add more oil to a pan and fry it.