- Preheat the oven to 200°C/180°C fan.
- Take the peas out of the freezer and let them defrost.
- Peel the potatoes and cut into chips. Put the chips into a pan of salted water, bring to the boil, and allow to simmer for 3 minutes. The potatoes should be tender but still hold their shape. Drain, and leave to steam dry for 5-10 minutes.
- Chop the onion. Melt the butter in a saucepan and add the onion. Cook for 5 minutes or until the onions are soft. Chop the parsley, dill and capers. Zest the lemon and juice it.
- Stir in the flour, then add the milk a little at a time, stirring as you add to make a smooth sauce. Bring to a simmer, stirring all the time as the sauce thickens (the sauce should be the consistency of double cream). Then add the parsley, dill, capers and lemon zest and juice. Season to taste with some salt and pepper.
- Remove skin from the cod if necessary and cut it into chunks.
- Spread half the sauce over the base of a shallow ovenproof dish, scatter the fish and peas on top and then top with the remaining sauce.
- Place the cooked chips in a bowl, toss with the vinegar, a little vegetable oil and some salt.
- Scatter the chips over the pie and bake for 35-40 minutes in the preheated oven until the chips are golden and crispy. Remove from the oven, let the pie rest for 5 minutes and then serve with lemon wedges.