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Soak the potatoes in cold water for 1 hr, drain, then cut into quarters. Cook in boiling salted water for 5 mins, drain and set aside until cool enough to handle. Coarsely grate and transfer to a bowl.
Stir in the onion, garlic, flour, egg and plenty of seasoning. Divide the mixture into 24 and shape into small patties (known as latkes). Heat a little of the butter and oil in a large non-stick frying pan and fry the latkes over a medium heat for 3-4 mins each side until golden brown (you will need to do this in batches, add more butter and oil each time). Drain on kitchen paper and keep warm.
Mix the crème fraîche, herbs and horseradish sauce together and season. Spoon the mixture into a serving bowl. Slice the smoked salmon into strips and break the smoked mackerel into bite-sized pieces. Arrange on a serving platter with the potato latkes, herby crème fraîche and some lemon wedges.