Scrub and peel 500g of potatoes – I like to use the waxy, yellow-fleshed varieties for this, but any potato will work – then slice no thicker than a pound coin. Preheat the oven to 190C/gas mark 5.
In a large jug, mix together 300ml of double cream and 200ml of full cream milk, Add salt, ground black pepper, 1 tbsp of grain mustard and 3 bay leaves, Pull the leaves from a few bushy little sprigs of thymeenough to give you 1 tbsp of leaves, then roughly chop them and stir them in.
Remove the skin from 225g of smoked mackerel and discard it, then break the fish into small pieces. Let them break naturally into short lengths, about 3cm long. It is worth being scrupulous about the bones: check each piece of fish very carefully, pulling out any you find.
Using a large, shallow baking dish (I use one 20-22cm in diameter) layer the potatoes in the dish, tucking in pieces of smoked fish every now and again. Pour over the cream mixture and bake for 55-60 minutes.
Test the potatoes with a metal skewer – it should slide through easily. If not, return to the oven for a little longer. Enough for 3-4
There is no need to be too pedantic about the size of the mackerel pieces, but I feel they are most delicious when kept quite large and juicy, about 3cm in length. Finding large pieces of fish among the potatoes is more pleasing than fish that has been mashed to a paste.
Give the finished dish a good 10-15 minutes to calm down before bringing it to the table. Some of the cream will soak into the potatoes as they cool, making them crumbly and tender.
The Observer aims to publish recipes for seafood rated as sustainable by the Marine Conservation Society’s Good Fish Guide