Cut the potatoes into large chunks and cook them in lightly salted water for 12-15 minutes or until tender. Drain under cold water and peel. Mash the potatoes and place them in a bowl. Add the fish, egg, scallion, olives, tomatoes, parsley, mustard and salt and pepper to taste. Mix the ingredients to distribute them evenly. Shape portions of the mixture into 4-8 patties about 1/2-thick. Dredge the patties in the flour and shake off excess. Heat the butter and olive oil in a sauté pan over medium heat. When the butter has melted and looks foamy, add the patties and cook for about 3 minutes per side or until hot, crispy and golden brown. Serve garnished with lemon quarters.