In a pan, cook the potatoes for 20 minutes over a moderate heat until mashable, drain and keep warm.
Add the milk, cream, garlic, bay leaf and thyme to another pan, bring to the boil and then remove from the heat to infuse for 25 minutes. Add the cod and bring back to simmer for 5-7 minutes, then cool. Remove the cod retaining the liquid.
In a clean pan mash the potato and the cod together with a fork and add the olive oil and use some of the retained milk to create a soft texture. Transfer into a gratin dish, top with the cheese, breadcrumbs and olives and finish in the oven at 190°C for twenty minutes until brown and crisp.