Preheat the oven to 200°C/ fan forced 180°C.

Peel the potatoes and cut them across into 1cm (½inch) thick slices.

Heat the extra virgin olive oil and sliced garlic in the casserole, as soon as the garlic starts to sizzle, add the tomatoes and cook for 4 minutes until soft. Stir in the potatoes, chicken stock, thyme, saffron and ½ teaspoon of salt. Cover with the lid and simmer gently for 20 - 25 minutes, carefully turning the potato slices over half way through. Cook until the potatoes are almost tender and the liquid is partly-reduced.

For the harissa mayonnaise; put the egg yolk, lemon juice and salt into a small mixing bowl. Using an electric hand held whisk or wire whisk, beat in the harissa and garlic, then very gradually drizzle in the oil, beating all the time, until you have a thick, mayonnaise-like mixture.

Brush the pieces of cod fillet on both sides with some of the melted butter, then season well with salt and pepper. Heat a non-stick frying pan over a high heat. Add the pieces of fish, skin-side down and cook for 4 minutes until nicely golden. Turn the fish over and cook for 2 minutes.

Uncover the potatoes and place the fish on top, skin-side down. Place the casserole into the oven and roast for 5 - 7 minutes, or until the fish is just cooked through.

Carefully lift the cod onto a plate and spoon some of the potatoes into the centre of 4 warmed soup plates. Place the cod on top, garnish with a few thyme leaves and serve with a spoonful of the harissa mayonnaise.