Wipe the inside of the skillet clean and heat 3 tablespoons of the olive oil over moderately low heat. Add the carrot, onion, celery, fennel and garlic and cook, stirring, for 2 minutes. Pour in the sherry vinegar and reduce until nearly all of the liquid has evaporated. Add the white wine, bouquet garni and sugar and season with salt and pepper. Cook for 2 minutes, then stir in the ketchup. Pour in the remaining 1/2 cup plus 3 tablespoons of olive oil and cook for several minutes more, until the vegetables are still slightly crunchy. Pour the hot marinade over the mackerel, transfer to the oven and bake for 3 to 4 minutes. Remove from the oven and cool to room temperature.