Preheat the oven to 180°C. In a small bowl mix all the horseradish cream ingredients together and taste for seasoning, adding more salt, pepper or lemon juice if you think it needs it.
Mix all the avocado relish ingredients together starting with the juice of half a lemon and only adding more if you think the relish needs sharpening up a little. Set aside. Cut the pittas open along the long edge with a sharp knife and then cut each one in half across the middle. Toast the pittas in the oven until golden brown all over, about 5 minutes, watching them like a hawk as they burn very easily! Remove from the oven and immediately rub the garlic gently over the warm toast to give it the slight hint of garlic.
Break up the mackerel into small chunks and mix it into the horseradish cream. If needed, season with a little more lemon juice and a good sprinkling of chopped parsley. Stuff the mackerel into the pittas and top with the avocado relish. Serve at once whilst the pittas are still crisp.