While the potatoes are cooking, take a deep, generously-proportioned ovenproof dish and stand a grater in it. Peel the carrot and, using the coarse side of the grater, grate the carrot, along with the celery and Cheddar cheese into the dish. Then, using the fine side of the grater, grate the zest from the lemon over the mix. Finely chop the parsley and add to the dish.
Cut the fish into bite-sized chunks (ensure there are no bones) and add them to the dish. Add any prawns or shellfish that you may wish to include. Squeeze the juice from the lemon over the mixture then drizzle with olive oil and a good pinch of salt and freshly-ground pepper. Mix well then spoon the crême fraîche over the top.
Spread the Creamed Potatoes evenly over the top of the fish mixture (you can create pretty patterns with a fork if you want to impress!). Cook in the middle of a preheated fairly hot oven (Mk 6 - 400ºF - 200ºC) for 40 minutes until cooked through, crispy and golden on top. Serve steaming hot with vegetables of your choice.