Place potato in a large saucepan. Cover with cold water. Bring to the boil over high heat. Boil for 12 minutes or until tender. Drain. Return to pan over low heat. Toss potato until liquid has evaporated. Roughly mash. Transfer to a bowl. Set aside to cool.
Drain mackerel, ad onions, green pepper and season with salt and pepper. Stir to combine. Using damp hands, shape mixture into 4 patties.
Coat the patties in breadcrumbs, shaking off excess. Place on a plate lined with baking paper. Refrigerate for 20 minutes or until firm.
Meanwhile, make Tomato and Cannellini Bean Salad: Combine tomato and lettuce in a bowl. Drizzle over dressing. Season with salt and pepper. Toss gently to combine.
Pour enough oil into a large non-stick frying pan to come 1cm up side of pan. Heat over medium heat. Shallow-fry patties for 2 to 3 minutes each side or until golden and crisp. Drain on paper towel. Serve patties with salad.