For the mackerel, heat the olive oil and butter in a frying pan. Add the mackerel and sprinkle with rosemary, salt and freshly ground black pepper. Fry for two minutes on each side, or until cooked through. Squeeze over the lemon juice and remove from the heat.
For the dressing, place the olives, garlic and olive oil into a mini blender and process until smooth.
To serve, place the mackerel onto a serving plate. Pour over the dressing and garnish with fresh basil leaves.
