Preheat the oven to 450°F/230°C. Prepare a baking tray with cooking spray. Set up a breading station with seasoned flour, beaten egg and bread crumbs.
Mix the fish, shrimp, onion, parsley, salt and pepper together in a large bowl. Add a tablespoon of milk if the mix doesn't hold together. Taste and adjust the seasoning before forming them into small balls or oblong shapes. You should have two dozen, depending on the amount of mix you use.
After the croquettes have been formed, bread them by dipping first into the flour, then the egg, then the bread crumbs. Arrange on the prepared baking sheet.
Spray the tops of the croquettes with cooking spray and transfer to the hot oven.
Bake for 20 - 30 minutes or until they take on a nice color. Flip the croquettes once or twice and apply more cooking spray as needed.
Serve with your favorite tartar sauce, ketchup and aioli