Bring a pan of lightly salted water to a boil. Add the potatoes and cook until almost tender, about 10 minutes. Drain and rinse under cold running water to stop the cooking. Drain well and pat dry with paper towels. Using the largest holes on a box grater-shredder, grate the potatoes into a large bowl. Add the flour, onion, ½teaspoon salt, and ¼teaspoon pepper and mix well with a fork. Stir in the egg. With lightly floured hands, form the mixture into walnut-sized balls, rolling them between your palms, then flatten the balls into patties.
Cook the pancakes
In a small bowl, combine the oil and butter. Place a large frying pan over medium heat and brush generously with the oil-butter mixture. Working in batches, cook the pancakes until golden and crisp, 4–5 minutes per side. Transfer to a paper towel–lined plate. Top each pancake with a dollop of the crème fraîche and a few slivers of salmon. Arrange on a platter, sprinkle with the chives, and serve.