Preheat the oven to 500°F. Line a roasting dish with parchment paper.
Put 2 tbsp of oil into a small saucepan and place over medium-low heat. Add the garlic and cook for 10 seconds, then add the cilantro, fish spice mix, chile flakes, ¼ tsp of 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 cod in the prepared roasting dish, skin side down, and brush with the remaining 2 tbsp of oil. Season lightly with salt and pepper, then spoon the cilantro 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 2 slices of lemon. Roast for 7–8 minutes, or until the fish is cooked through. Serve at once, with about 1 tbsp of tahini sauce drizzled over, if desired, and lemon wedges alongside.