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Boil the eggs for 7 mins, then plunge straight into cold water and set aside to cool.
Melt the butter in a large saucepan and add the oil and onions. Cook for 5 mins or until soft. Stir in the flour for 1 min to make a paste, then add the milk bit by bit, stirring as you go, to make a smooth sauce the consistency of double cream. Add the parsley, cornichons, capers and lemon juice, and season to taste.
Heat oven to 200C/180C fan/gas 6. Peel the potatoes, cut into chips, put in a pan of water. Bring to a simmer, cook for 2 mins, then drain – the potatoes should still hold their shape. Leave to steam-dry for 5 mins.
Spread half the sauce over the base of a large shallow casserole dish (ours was 30cm). Scatter the fish on top, peel and quarter the eggs, and add these too. Top with the remaining sauce. In a bowl, toss the chips with the vinegar, a drizzle of oil and some seasoning. Scatter over the pie and bake for 40 mins until the potatoes are golden.
Meanwhile, cook the peas in a pan of boiling water for 2-3 mins, then drain and mash lightly with a knob of butter and the lemon zest. Serve alongside the pie.