Preheat the oven to Gas Mark 4, 180°C/fan 160°C (350°F).
Put the potatoes, milk, cream, garlic, nutmeg and a little salt and pepper in a large saucepan and bring to the boil. Reduce the heat and simmer for 3 minutes, stirring regularly.
Remove from the heat and transfer the potatoes and sauce to an ovenproof baking dish. Cover with foil and bake for 40 minutes. Remove the foil and bake for a further 15 minutes until the top is golden.
To cook the hake: Preheat the grill to hot. Place the fish, flesh side up, on a baking sheet which has been lined with foil and oiled, brush with a little softened butter and season with salt and pepper. Grill for about 8-10 minutes until the fish flakes easily.
Meanwhile place all the ingredients for the Sundried Tomato Dressing in a food processor and whizz until puréed.