Boil potatoes until tender, drain and reserve.
Return the pot to the stovetop over medium heat and add butter and orange juice. Cook 1 minute then add stock and brown sugar. Add reserved potatoes back to the pot and mash until combined. Season with nutmeg, salt, pepper and orange zest, to taste.
Pour the potatoes out into a 9x9” baking dish and layer the banana slices over the top, overlapping them like shingles. Sprinkle the top with sugar and broil until caramelized.