Peel, slice and boil potatoes; drain and set aside.
Heat saute pan, add vegetable oil and one tablespoon margarine and use to saute the chopped onion, escallion, sweet peppers, 2 cloves garlic, diced tomatoes and hot pepper.
Fold in the mackerel, corn and black pepper and simmer for 3 minutes for flavours to develop.
Combine the breadcrumbs, parsley and one tablespoon melted margarine and mix well, then set aside.
Assembling the au gratin:
Layer sliced boiled sweet potatoes at the base of a greased casserole. Spread the mackerel mixture over the layered potatoes.
Sprinkle the crumbled feta cheese over the mackerel mixture. Top with the parsley and breadcrumbs mixture.
Place in a preheated oven 180°C/350°F and bake until a golden brown colour develops.
Cut the green escallion leaves into strips and use to tie the carrots and string beans into bundles.
Heat the vegetable oil or margarine; add the remaining garlic and saute. Add the carrot and string bean bundles and saute for one minute.
To Plate: Spoon the mackerel and potato mixture unto the serving plate. Serve with sauteed bundles of carrots and string beans.
Note: Cheddar cheese may be used in place of feta cheese.