Peel the potatoes and cut into large even-sized chunks, then cook them in a large pan of boiling salted water for 15 minutes, or until soft, adding the peas for the last 2 minutes. Drain and mash well with a pinch of sea salt and freshly ground black pepper, the zest from the lemon and the butter. Set aside.
Peel and chop the carrots and onions, then fry them in a large ovenproof pan (roughly 30cm in diameter) with 1 tablespoon of oil over a medium heat for 15 minutes, or until softened but not coloured, stirring occasionally.
Meanwhile, heat the milk in a pan on a medium heat. Once simmering, add all the frozen fish fillets and cook for around 10 minutes, or until cooked through, then use a slotted spoon to transfer them to a plate, taking the pan off the heat.
Stir the flour into the carrots and onions, then gradually add the warm milk, a ladleful at a time, stirring continuously until you have a smooth sauce. Stir in the spinach until thawed and incorporated, then season to perfection.
Flake in the cooked fish fillets (carefully remove and discard any skin or bones), add the prawns and mustard, along with the juice of half the lemon, grate in the Cheddar, and stir gently to combine.
Top with the pea-spiked mash and smooth out, scuffing it up slightly with a fork to give it texture that will crisp up. Bake for 30-40 minutes, or until beautifully golden. Serve with a good old helping of baked beans (if you like). Delicious!
