Preheat the oven to 250°C fan.
To make the tahini sauce: Mix together all the ingredients and put aside.
Put a glug 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, salt and a grind of black pepper. Cook for 4–5 minutes, stirring frequently, for the garlic to really soften, then remove from the heat.
Place the fish in a baking paper-lined roasting dish, and brush with more olive 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 along with 2 slices of lemon. Roast for 7–8 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.