Start by cooking the fish until flaky, then allow it to cool.
In a bowl, combine the cooked fish and mashed potatoes.
Add chopped green onions, mayonnaise, and Dijon mustard.
Mix in the egg, and season with salt and pepper.
Shape the mixture into patties about 1 inch thick.
Coat each patty with bread crumbs for extra crunch.
Heat oil in a skillet over medium heat.
Fry the fish cakes until golden brown on both sides, about 4-5 minutes each.
Remove from the skillet and drain on paper towels.
Serve hot with lemon wedges.