Set out one large, clean plate or platter and one medium plate. Pour panko breadcrumbs onto medium plate.
Place cooked fish into a large bowl and flake into small pieces with a fork. Add sour cream, egg, celery, capers, green onion, salt, and pepper; stir to combine. Scoop out 1/8 of the mixture and use your hands to gently pack into a patty. Place on plate with panko crumbs and gently coat all sides with crumbs; transfer to platter. Repeat with remaining mixture.
Heat 2 tablespoons oil in a large skillet over medium heat until hot. Place 4 fish cakes in skillet and cook until crumbs are crisp and golden brown, pressing down gently with spatula to compress; about 5 minutes per side (turn gently). Transfer cooked fish cakes to warm platter. Repeat process with remaining oil and fish cakes.
Serve fish cakes hot with lemon wedges if desired.
Video
Notes
You can make the fish cake mixture up to one day before you plan to form and pan fry the fish cakes.
This recipe works well with cod and halibut, but is also great for leftover salmon.
If you'd like a dipping sauce, I recommend either this spicy, Louisiana-style Remoulade sauce or French Remoulade Sauce, which is closer to tartar sauce.