Preheat the oven to 200C. Start with the potatoes. Bring a large pot of salted water to boil and add the peeled, quartered potatoes to cook for 20-25 minutes, until soft.
In the meantime, place the cod loins in an ovenproof dish and add 1 litre of milk, or enough milk to just cover them. Add 2 teaspoons of salt, a teaspoon of black pepper, a chopped bunch of fresh parsley and dill and chopped spring onions. It needs to be submerged and soaked in the milk (otherwise it will burn). Place the whole dish in the oven and the fish will poach in 20 minutes.
Fry some rashers of streaks bacon until crispy.
Once ready, drain the potatoes and add them back to the pot you cooked them in. Add a tablespoon of butter to melt over the potatoes.
After 20 minutes, remove the fish from the oven. Carefully pour out the poaching milk into a jug, without taking any of the fish or herbs. A sieve or a colander might help. Pour roughly 150-200ml of the poaching milk into the buttered potatoes and mash until you have a smooth, creamy mash.
Take the chopped herbs and spring onions left behind in the fish-poaching dish and stir them through the mashed potatoes. Also snip up some of the crispy-fried bacon into the mashed potatoes.
Take one of the cod fillets and gently break it up with your fingers into the mashed potatoes. Gently fold it into the mashed potatoes, keep it as big flakes. Leave it to stand with a lid on..
To a saucepan, add a tablespoon of cornflour and add a tablespoon of the leftover poaching milk. Mix to a paste before gradually adding the rest of the milk. Heat until it is thick.
Serve mash with extra fish flakes, bacon and gravy.