Prick the potatoes all over with a fork and place on a plate. Cook in the microwave oven for 8 minutes, turning over once, until tender when pierced with the tip of a knife. Let sit for 5 minutes. Cut the potatoes in half, scoop out the flesh with a spoon and place in a large bowl. Coarsely mash with a fork. You will need 2 cups (500 ml) of mashed potatoes.
Combine the potatoes with the mackerel, egg, parsley and lemon zest. Generously season with salt and pepper.
Place the breadcrumbs in a shallow dish.
Form the potato mixture into twelve patties, using about ¼ cup (60 ml) for each one. Press into the breadcrumbs and coat well.
In a large non-stick skillet over medium heat, cook half of the patties at a time in the oil for 3 minutes on each side or until nicely golden.
In a bowl, combine all of the ingredients.
Serve the mackerel croquettes with the tartar sauce and lemon wedges. Serve with a green salad, if desired.