Soak the herring fillets in milk overnight or for 12 hours. Drain and discard the milk.
Lay the herrings in a clean, shallow dish, pour over the oil and add the spices, bay leaves and thyme. Place the onion and carrot in a layer on top, cover and leave to marinate in the fridge for 12 hours.
Cook the potatoes in their skins for 18 minutes or until tender. Cool and cut into 1cm (