Grease the base of a tagine or large overproof dish with 1 tablespoon of the oil.
Arrange most of the tomato slices over the base and sprinkle over half the garlic and chillies. Place the fish in a single layer on top of the tomatoes, scatter over the remaining garlic and chillies and arrange the remaining tomato slices on top of the fish.
Mix the remaining olive oil together with the lemon juice in a small bowl. Pour the mixture over the fish and tomatoes and season. Cover and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes.
Scatter the butter over the tomatoes and sprinkle the cinnamon and sesame seeds over the top. Return the tagine to the oven, uncovered, for 10 minutes until the fish is cooked through. Serve with couscous and a salad, if liked.