Make the tahini sauce (makes one medium jar): Mix together all the ingredients, along with 120ml of water and quarter teaspoon of salt. If it is too runny, add a bit more tahini. If it is too thick, add a bit more lemon juice or water. You want the consistency to be like that of a smooth, runny nut butter. It will thicken up when left to sit around, so just give it a stir and some more lemon juice or water every time you use it.
To make the fish spice mix simple stir all the ingredients together.
Preheat the oven to 230°C fan.
Put two tablespoons of oil into a small saucepan and place on a medium-low heat. Add the crushed garlic and cook for 10 seconds, then add the coriander, fish spice mix, chilli flakes, a quarter teaspoon of salt and a grind of black pepper.
Cook for four to five minutes, stirring frequently, for the garlic to really soften, then remove from the heat.
Place the cod in a parchment-lined roasting dish, skin side down, and brush with the remaining two tablespoons of oil. Season lightly with salt and pepper then spoon the coriander mix on top of each fillet. Spread it out so that the whole top is covered, then top each one with a bay leaf, if using, along with two slices of lemon. Roast for seven to eight minutes, or until the fish is cooked through.
Serve at once, with about a tablespoon of tahini sauce drizzled over, if using, and a wedge of lemon alongside.