Preheat the oven to 200°C/400°F/gas 6 and bring a large pan of salted water to the boil. Peel the potatoes and cut into 2cm chunks. Once the water is boiling, add your potatoes and cook for around 12 minutes, until soft (you can stick your knife into them to check).
Meanwhile, get yourself a deep baking tray or earthenware dish and stand a box grater in it. Peel the carrot. Grate the celery, carrot and Cheddar on the coarse side of the grater. Use the fine side of the grater to grate the zest from the lemon. Finely grate or chop your chilli. Finely chop the parsley leaves and stalks and add these to the tray.
Cut the salmon and smoked haddock into bite-size chunks and add to the tray with the prawns. Squeeze over the juice from the zested lemon (no pips please!), drizzle with olive oil and add a good pinch of salt and pepper. If you want to add any spinach or tomatoes, do it now. Mix everything together really well.
By now your potatoes should be cooked, so drain them in a colander and return them to the pan. Drizzle with a couple of good lugs of olive oil and add a pinch of salt and pepper. Mash until nice and smooth, then spread evenly over the top of the fish and grated veg. Place in the preheated oven for around 40 minutes, or until cooked through, crispy and golden on top. Serve piping hot with tomato ketchup, baked beans, steamed veg or a lovely green salad