- Preheat the oven to 180°C (160° fan)/ gas 4.
- Arrange the mackerel fillets in an oval baking dish or ovenproof pan.
- Whisk together the olive oil, lemon juice, chopped garlic, and diced tomatoes in a mixing bowl. Season with some freshly ground black pepper.
- Spoon the mixture over the fish. Bake for 15-20 minutes until the mackerel is firm to the touch and opaque in appearance.
- Remove from the oven and let stand briefly before sprinkling with some chopped parsley and serving with lemon wedges and the baguette slices on the side, if desired.
- Serve with a mixed salad instead of bread for a low-carb version.