Celeriac is a very underused and underrated vegetable although it is often used in Polish kitchens in soups and in salad recipes. It actually has a very mild, sweet flavour and makes a wonderful alternative to raw cabbage in a slaw.
Start by making the celeriac slaw. Take a large bowl and fill it with cold water. Add the lemon juice and set to one side.
Carefully peel the celeriac. Since the outer edge of the celeriac can be tough, you may need to cut away the exterior with a sharp knife. Using the thicker side of a box grater, grate the celeriac. Tip straight into the lemon water, as it can quickly discolour. Finely slice the green apple into matchstick-sized pieces and place the apple pieces into the water with the celeriac. Peel and grate the carrot, too.
Drain the celeriac and apple into a colander. Make sure you squeeze most of the lemon water out. Tip it into a bowl. Add the carrot. Stir though the raisins and the sour cream. Set to one side while you cook the fish.
Chop the fish into smaller fish-finger-sized pieces. Add the oil to a large non-stick pan and heat gently. Dip the fish pieces, one at a time, into the egg and then into the flour to lightly coat them. Place each fish finger into the pan and fry gently for 3-4 minutes on each side. You may need to flip them over a couple of times and take care that they do not burn. Serve the fish fingers immediately with the slaw.