Place the potatoes in a large pot of boiling water or in a steamer and cook until soft.
Pour boiling water over the peas to de-frost them then blend in a food processor.
Mash the potatoes then mix in peas, butter and lemon zest. Season to taste.
Pre-heat the oven to 200°c.
To make the fish pie, poach the fish in the milk with the bay leaf. When the fish is cooked, remove the fish and flake into large chunks. Reserve the milk.
In a large, oven-proof frying pan fry the onion and carrot in a splash of olive oil until soft and fragrant. Add the garlic and fry for another 30 seconds.
Add the flour and stir then add the milk the fish was poached to create a creamy sauce.
Add the English mustard and fish and stir well then add the cheese and lemon juice and stir.
Season to taste.
Top the fish filling with the mashed potato and create indents with a spoon which will become nice and crispy in the oven.
Place the pie in the oven and allow to bake for 30-40 minutes until the top is golden brown and crispy.
Remove from the oven and allow to cool for 10 minutes then serve.