Preheat oven to 400F and bring a large pot of salted water to boil.
Peel potatoes and cut into 3/4-inch chunks. Once water is boiling, add your potatoes and cook for around 12 minutes, until soft (you can stick a knife in them to check).
Meanwhile, get yourself a casserole dish (9"x13") and stand a box grater in it. Peel the carrot and grate the carrot, celery and cheddar on the coarse side of the grater. Use the fine side to zest the lemon. Finely grate or chop your chile. FInely chop the parsley leaves and stalks and add them to the dish.
Cut the salmon and cod into bite sized chunks and add to the dish, along with the shrimp. Squeeze over the juice of the zested lemon, drizzle with olive oil and add a good pinch of salt and pepper.
If you want to add the spiach and/or tomatoes, do it now. Mix everything together really well.
By now, you potatoes should be cooked, so drain them in a colander and return them to the pot. Drizzle with a couple of glugs of olive oil and add a pinch of salt and pepper, to taste. Mash until nice and smooth, then spread evenly over the top of the fish and grated vegetables.
Place in the preheated oven and bake for around 40 minutes, or until cooked through ans crisp and golden on top. You can turn the broiler on for a couple of minutes at the end to help achieve this effect.
Serve piping hot with tomato ketchup and/or baked beans, if you're feeling especially English. Or with a green salad. Or just, as is.