Fry the bacon in a hot pan until it is crispy. Add the celeriac and turnip and sizzle until the vegetables start to become tender. Transfer the mixture to a bowl and add the apple, carrot and parsley.
Brush a bit of mustard onto the inside of the fish and sprinkle with salt. Fry in a neutral type of oil in a hot pan with the meaty side facing downwards until it turns a light golden brown color and the meat falls off the bone, usually around 7 minutes.
Arrange the fish on the plate with the bacon, vegetable and apple mixture on top.