Peel the potatoes and cut into 2cm chunks, then boil for around 15 minutes, or until tender. Add the eggs for the last 8 minutes (if using).
Meanwhile, make the sauce. Melt the butter in a heavy-bottomed pan over a low heat and stir in the flour. Add the bay, then bit by bit, add the stock and the milk, stirring after every addition until you have a smooth, silky sauce. Bring to the boil, then reduce to a simmer and cook for 10 minutes, or until thickened.
Stir in the mustard, grate in half the cheese, then squeeze in the lemon juice. Pick, finely chop and stir in the parsley (if using), then keep stirring until the cheese is melted. Season with a little black pepper, then remove from the heat.
Get yourself a 25cm x 30cm baking dish. Check that there aren’t any stray bones lurking in the fish fillets, then slice into 2.5cm chunks and spread them evenly over the base of the dish.
Wilt and add the spinach, then peel, quarter and add the eggs (if using). Remove the bay leaves, then pour over the white sauce and allow to cool slightly.
Drain the potatoes well and mash with a little extra virgin olive oil, a splash of milk and a few scrapings of nutmeg. Spoon the potatoes over the pie and scuff up the surface with a fork.
Grate the remaining Cheddar over the top and bake in the oven for 45 minutes, or until the fish is cooked through, the sauce is bubbling up at the sides and the top is golden.
Delicious served with lots of green veg, or if you’re Jamie, with baked beans!