Combine the grated onion and potato in a kitchen towel and wring out as much liquid as you can. Pour the mixture into a mixing bowl. Dust with the flour and mix to disperse it. Add the egg, lemon zest, chopped dill, salt and pepper and mix until well combined.
Heat the olive oil and butter in a large nonstick saute pan over medium-high heat. Once the bubbles from the butter subside, spoon 3 heaping tablespoons of the potato mixture into the pan. Repeat, making 4 to 8 pancakes. Cook the pancakes about 4 minutes on each side, assuring that both sides are well browned and the potato is cooked through. Reduce the heat of the pan if the potato cakes are browning too quickly in order to make sure the inside is cooked completely.
Remove the cakes to a paper-towel-lined plate as they finish and season with salt while hot.
(You can cover them with foil and keep in a 200 degrees F-oven to stay warm until ready to serve.)
To serve, transfer the potato pancakes to a platter and dollop each one with the creme fraiche. Top with a sprig of dill.