Preheat the oven to 200°C (400°F) and bring a large pot of salted water to a boil.
Peel the potatoes and cut them into 2 cm cubes.
Once the water is boiling, add the potatoes and cook for about 12 minutes, until tender (check with a knife).
In the meantime, take a deep baking dish or a ceramic plate and place a grater on it.
Peel the carrot. Grate the celery, carrot, and cheddar on the rough side of the grater.
Use the smooth side of the grater to grate the lemon zest.
Grate or finely chop the red pepper.
Finely chop the parsley leaves and stems and add them to the dish.
Cut the smoked salmon and haddock into cubes and add them to the dish, along with the prawns.
Squeeze the lemon over the dish (without any pips), drizzle with a little olive oil, and add a good pinch of salt and pepper.
Add spinach or tomatoes at this point, if desired.
Mix well.
At this point, the potatoes should be ready. Drain them in a colander and return them to the pot.
Drizzle with two or three good drizzles of olive oil, add a pinch of salt, and pepper.
Mash until smooth, then spread over the fish and grated vegetables.
Put in the preheated oven for about 40 minutes, or until cooked through, when it’s crispy and golden.
Serve straight from the oven with tomato ketchup, cooked beans, steamed vegetables, or a nice green salad.