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Mix together the mustard, horseradish and crème fraîche in a small bowl. Tip the celeriac and celery into a large bowl, add half the crème fraîche mixture, and season well. Chill in the fridge until needed.
Heat the grill to high and grill the bacon until crisp, turning once during cooking.
Put the haddock on a plate, add the flour and season with black pepper. Toss so that the fish pieces are well coated. Heat the oil in a large, non-stick frying pan and cook the fish for around 4 mins on one side. Turn over and cook for about 2 mins more or until lightly golden on the outside and just cooked in the middle. You may want to cook this in 2 batches so that you don’t overcrowd the pan. Transfer the fish to a plate to rest.
Meanwhile, mix the lemon juice into the celeriac mixture and pile it onto plates, with a rasher of bacon and a piece of haddock. Scatter over the dill, and serve immediately with the lemon wedges and the remaining crème fraîche dressing on the side.