Heat the oven to 200°C/180°C fan-assisted/Gas Mark 6. Break up 1 large head of garlic into cloves leaving the skin on. Place in a piece of foil and wrap to cover. Roast in the oven for 20 minutes until tender. Unwrap the foil and leave to cool.
Meanwhile place the potatoes in a pan of cold water and bring to the boil. Boil for 15 minutes until tender then drain and mash with the milk. Squeeze the soft garlic out of the cloves discarding their skins. Fold through the mash and season.
Using a peeler peel thin layers of the zest from the lemon then finely shred. Then juice the lemon and set both aside.
Dust the fish in the flour and shake off any excess. Heat the oil in a large non-stick pan over a medium heat add the fish skin side down. Cook for 3 minutes before gently turning and cooking for a further 2-3 minutes until cooked through. Carefully remove from the pan and keep warm.
Keep the pan over the heat and add the butter, lemon zest and juice. Once melted season and add the fish stock and parsley. Simmer for 2-3 minutes while you spoon the mash onto the plates and top with the fish. Spoon over a little sauce and serve immediately.