Finely grate the potatoes on a hand grater. Place in batches into a clean kitchen towel and squeeze out as much liquid as possible, place in a large bowl. Add the onion, beaten egg, baking powder, flour, and cilantro, and mix to combine. Season with salt and pepper, to taste.
Heat 1/4 cup of vegetable oil in a large cast iron skillet until almost smoking. Drop potato mixture into the skillet 1 heaping tablespoon at a time and smooth the tops with the back of a spoon to 1 1/2-inches in diameter. Fry over moderate heat, turning them once with a spatula for 3 to 4 minutes on each side, or until golden brown and crisp.
Remove to a platter lined with paper towels. Repeat with remaining mixture, adding more oil if necessary. Top each with a small dollop of mango serrano creme fraiche, flaked trout, and a cilantro leaf.
Mango-serrrano creme fraiche:
Combine in a medium bowl. Refrigerate until ready to serve.