1) Preheat the oven to 350°F (180°C), Gas Mark 4.
2) In a bowl, combine drained gooseberries, tartare sauce and potatoes.
3) Take each mackerel to fill with some of the potato mixture.
4) Wrap the fish with aluminum foil.
5) In a ovenproof dish place the fish.
6) Place the dish inside oven to bake for 30-40 minutes.
7) Serve by un-wrapping the fish on an oval platter by garnishing with sliced tomatoes and sprigs of parsley.