Preheat the grill to high. With a sharp knife, slash the sides of the mackerel and season with salt and freshly ground black pepper.
Place on a grill tray and grill for 3-4 minutes on each side, or until cooked through.
Transfer the mackerel into a deep serving dish.
Cover the mackerel with the chopped tomatoes and capers.
Pour the mackerel juices from the grill tray over, along with the lemon juice. Set aside in a warm place.
Heat a frying pan until just warm. Add the oil, potatoes, peppers and garlic and stir to combine.
Add the white wine and simmer gently, stirring from time to time to prevent sticking, until the potatoes are cooked through and the peppers are soft.
Season with salt and freshly ground black pepper, to taste.
Serve the mackerel with the potatoes on the side.
