Preheat the oven to 200°C (400°F/Gas Mark 6).
Place the cod on a baking sheet, skin side down, and season the flesh. Arrange the tomato slices on top and tuck in the basil leaves. Cover with mozzarella and drizzle over the pesto.
Place the cod in the oven and bake for 12-15 minutes, until the cod is cooked through and the mozzarella is bubbling and lightly golden. Transfer the cod with cherry tomatoes, basil and mozzarella to warmed plates and have bowls of green salad and buttered baby boiled potatoes to hand around separately.