In a bowl, combine the onion and potato. Dust with the flour and mix to disperse it. Add the eggs, horseradish, lemon zest, fresh dill, salt, and pepper, and mix until well combined.
In a large non-stick saute pan, heat the olive oil and butter over medium-high heat. Once the bubbles from the butter subside, spoon 3 heaping tablespoons of the potato mixture into the saute pan. Repeat, making 8 to 12 pancakes. Cook 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 potato cakes to a platter and dollop each one with creme fraiche. Top with a sprig of dill.